🏷

Classifications

Harrison's Chapter

👪

Inheritance

1

Autosomal dominant inheritance HP:0000006

BLBS2 is inherited in an autosomal dominant manner and is typically caused by a de novo heterozygous H3-3B variant, so recurrence risk to siblings is low. No biallelic or recessive form has been described; the disease alleles are missense, synonymous (transcript-dependent) and stop-loss changes acting through gain-of-function or dominant-negative effects rather than loss of one copy.

Autosomal dominant inheritance

Show evidence (3 references)

PMID:37782742 SUPPORT Human Clinical

"BRYLIB is expressed in an autosomal dominant manner and typically caused by a de novo H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant."

GeneReviews assigns the disorder to autosomal dominant inheritance with a predominantly de novo mutational origin.

PMID:37782742 SUPPORT Human Clinical

"Therefore, the risk to other family members is presumed to be low."

States the low familial recurrence risk that follows from the de novo origin, which is what families are counselled on.

PMID:33268356 SUPPORT Human Clinical

"We analyze 46 patients bearing de novo germline mutations in histone 3 family 3A (H3F3A) or H3F3B with progressive neurologic dysfunction and congenital anomalies without malignancies."

The founding cohort establishes the de novo germline origin of the causative H3F3B variants.

◈

Mechanistic Hypotheses

1

H3.3 hyper-deposition model of BLBS2

h33_hyperdeposition_model EMERGING

Evidence balance 1 support

Proposes that the H3-3B disease allele acts by increasing H3-3B expression and driving excess incorporation of H3.3 into nucleosomes, so that the primary lesion is too much variant histone in chromatin rather than a loss of histone function. Demonstrated so far for a single allele (p.Leu48Arg) in one isogenic iPSC series; whether it generalises to tail-domain, core-domain and stop-loss BLBS2 alleles is untested, and it sits alongside the alternative view that different variants act through heterogeneous local mechanisms (impaired chaperone binding, altered protein stability, disrupted DNA contact).

Show evidence (1 reference)

PMID:41689096 SUPPORT In Vitro

"These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"

The hyper-deposition model rests on a single human-derived model system, which is why it is recorded as emerging rather than canonical.

?

Discussions and Knowledge Gaps

5

Why do variants in H3-3B produce a nosologically separate disorder (BLBS2) from variants in H3-3A (BLBS1) when the two genes encode an identical protein?

KNOWLEDGE GAP OPEN blbs2_paralog_basis

Attached to

pathophysiology#De Novo H3-3B Variant Altering Histone H3.3

The two BLBS entities exist because two genes are involved, not because two proteins are. The only known differences are in untranslated and coding sequence, which alter expression pattern and level. Cohort data show gene-dependent frequency differences (seizures 59% vs 45%; genital anomalies 28% vs 17%; delayed walking 65% vs 85%), so the paralogs are not interchangeable, but the mechanism converting a paralog-level expression difference into these phenotype shifts is unknown. Resolving it determines whether BLBS1 and BLBS2 should remain separate entities or be lumped.

Proposed experiments

Paralog-resolved H3-3A/H3-3B expression atlas across human brain development

exp_blbs2_paralog_expression_atlas

Quantify H3-3A and H3-3B transcript and protein abundance across human developmental brain stages and cell types to test whether the paralogs differ in the windows and lineages relevant to the divergent phenotypes.

Would support

Paralog-specific expression differences in neural lineages would explain why an identical protein product yields two clinical entities.

Would refute

Indistinguishable paralog expression across all relevant stages and cell types would argue that the BLBS1/BLBS2 split is nosological rather than biological.

Isogenic paralog-matched substitution series in iPSC-derived neurons

exp_blbs2_isogenic_paralog_swap

Generate isogenic iPSC lines carrying the same amino-acid substitution introduced into H3-3A versus H3-3B, then compare chromatin accessibility, transcriptome and neuronal electrophysiology between the matched pairs.

Would support

Divergent chromatin or electrophysiological phenotypes from the same substitution in the two paralogs would establish a paralog-intrinsic effect.

Would refute

Identical phenotypes from matched substitutions would locate the difference in mutational opportunity or ascertainment rather than paralog biology.

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Since these variables do not account for all BLBS phenotypic variability, these findings suggest that additional factors may play a role in modifying the phenotypes of affected individuals."

The authors explicitly state that gene, variant location and sex leave BLBS phenotypic variability unexplained.

Do all BLBS2 alleles act through a single mechanism (H3.3 hyper-deposition), or do tail, core and stop-loss variants act through distinct local mechanisms?

KNOWLEDGE GAP OPEN blbs2_variant_heterogeneity

Attached to

pathophysiology#Disrupted Nucleosome Assembly and Chaperone Interaction

Functional data point in different directions: some constructs reduce H3.3 protein levels while another accumulates above wild-type; one increases chaperone binding; and the one allele studied in a human neural model (p.L48R) increases H3-3B expression and nucleosomal H3.3. Whether these are variations on one theme or genuinely different mechanisms determines whether a single therapeutic strategy could serve the whole disorder.

Proposed experiments

Multi-omic iPSC allelic series across tail, core and stop-loss BLBS2 variants

exp_blbs2_allelic_series_multiomic

Apply the p.L48R iPSC pipeline to a panel of BLBS2 alleles spanning the histone tail, the histone core and stop-loss classes, quantifying nucleosomal H3.3 occupancy genome-wide alongside transcriptome and chromatin accessibility.

Would support

Shared hyper-deposition across allele classes would establish a single unifying mechanism and a single therapeutic target.

Would refute

Opposite-direction deposition changes across allele classes would confirm mechanistic heterogeneity and argue against a one-size-fits-all therapy.

Show evidence (1 reference)

PMID:34876591 SUPPORT In Vitro

"Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."

Demonstrates opposite-direction effects on H3.3 abundance among BLBS alleles, which is the substance of this gap.

Why do germline H3-3B variant carriers show increased cellular proliferation yet no excess of malignancy, including at residues (p.G34) that are oncogenic when mutated somatically?

KNOWLEDGE GAP OPEN blbs2_proliferation_no_cancer

Attached to

pathophysiology#Deregulated Cell Proliferation

Patient fibroblasts, including H3F3B p.G34V lines, proliferate faster than controls and up-regulate mitosis genes, yet the founding 46-patient cohort had congenital anomalies without malignancies and the community-wide cancer incidence across germline histonopathies sits at 1%. Somatic p.G34V is oncogenic only in combination with ATRX/DAXX and TP53 lesions, suggesting the germline context lacks the required cooperating hits, or that developmental timing and cell of origin gate transformation.

Proposed experiments

Longitudinal cancer-incidence follow-up of the international BLBS cohort

exp_blbs2_cancer_incidence_followup

Prospectively track tumour incidence in the registry cohort with standardised ascertainment, extending the community-wide incidence estimate and testing it against the 5% threshold used to justify routine surveillance.

Would support

An incidence rising above the surveillance threshold would establish BLBS2 as a cancer-predisposition syndrome.

Would refute

A stable incidence at or below population background would confirm that no tumour surveillance is warranted.

Cooperating-lesion transformation assay in germline H3.3-variant cells

exp_blbs2_cooperating_hit_transformation

Test whether germline H3.3-variant cells acquire transformation potential when ATRX/DAXX or TP53 are additionally disrupted, and compare the chromatin state of germline-variant glial precursors with K27M/G34-mutant glioma cells.

Would support

Transformation only in the presence of cooperating lesions would explain the germline proliferation-without-malignancy paradox.

Would refute

Failure to transform under any cooperating lesion would point instead to cell of origin or developmental timing as the gate.

Show evidence (2 references)

PMID:33268356 SUPPORT Human Clinical

"These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."

States the unresolved convergence between germline and somatic H3.3 mechanisms that motivates this gap.

PMID:40241305 SUPPORT Human Clinical

"the value of ongoing translational work to address the outstanding question of cancer predisposition for individuals living with germline histonopathies"

The pan-histonopathy cohort names cancer predisposition as an explicitly outstanding question.

Is BLBS2 genuinely neurodegenerative, or is the apparent regression a developmental plateau with musculoskeletal decline?

KNOWLEDGE GAP OPEN blbs2_neurodegeneration_extent

Attached to

pathophysiology#Abnormal Brain Development and Progressive Neurologic Dysfunction

BLBS was first described as a progressive neurodegenerative syndrome, and GeneReviews lists loss of milestones, worsening gait, spasticity and camptocormia. But the expanded cohort reframed the disorder as primarily neurodevelopmental, and some individuals show resolving rather than progressive hypotonia. No longitudinal imaging or biomarker study has established whether neuronal loss actually occurs. The answer changes both prognostic counselling and trial endpoint selection.

Proposed experiments

Prospective longitudinal MRI volumetry with neuroaxonal injury biomarkers

exp_blbs2_longitudinal_imaging_biomarker

Follow a BLBS2 cohort with serial MRI volumetry, standardised developmental assessment and serial neurofilament light chain measurement to distinguish genuine neuronal loss from a developmental plateau.

Would support

Progressive volume loss with rising neurofilament light chain would confirm a true neurodegenerative component.

Would refute

Stable volumes and biomarkers alongside a developmental plateau would reframe the apparent regression as non-degenerative.

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"BLBS was initially categorized as a progressive neurodegenerative syndrome caused by de novo heterozygous variants in either H3-3A or H3-3B"

Documents the initial neurodegenerative framing that the expanded neurodevelopmental characterisation now sits alongside.

PMID:40241305 SUPPORT Human Clinical

"histonopathy-associated phenotypes change throughout the lifespan, necessitating longitudinal re-evaluation"

Independent argument that the natural history is not yet settled and requires longitudinal study.

Does the available zebrafish evidence for BLBS reflect BLBS2 biology, given that it models the h3f3a paralog rather than h3f3b?

HUMAN MODEL MISMATCH OPEN blbs2_human_model_fidelity

Attached to

pathophysiology#Neural Crest and Craniofacial Developmental Defect

The craniofacial and glial arms of the mechanism rest substantially on a zebrafish h3f3a model (dominant p.D123N and homozygous db1092 alleles). Since the whole basis for splitting BLBS1 from BLBS2 is a paralog-level difference in expression rather than protein sequence, a model built on the other paralog cannot by construction test paralog-specific effects, and zebrafish h3f3 gene-family structure differs from the human two-gene arrangement. The craniofacial phenotype may therefore be attributable to total H3.3 dosage rather than to anything specific to H3-3B.

Proposed experiments

Orthologous h3f3b animal models for the craniofacial and glial arms

exp_blbs2_h3f3b_orthologue_models

Generate zebrafish and mouse models of orthologous h3f3b alleles, compare craniofacial and glial phenotypes with the existing h3f3a models, and quantify paralog-specific expression in neural crest across species.

Would support

Reproduction of the craniofacial and foxd3-glia phenotypes by h3f3b alleles would license the existing model evidence for BLBS2.

Would refute

Absence of the phenotype in h3f3b models would confine the current model evidence to total H3.3 dosage rather than to BLBS2.

Human iPSC-derived neural crest test of the foxd3-glia phenotype

exp_blbs2_ipsc_neural_crest

Differentiate BLBS2-allele iPSCs to neural crest and crest-derived glia to test whether the zebrafish foxd3-positive glial deficit reproduces in human cells.

Would support

A crest-derived glial deficit in human BLBS2 cells would bridge the cross-species gap.

Would refute

Normal human crest-derived glia would leave the zebrafish glial phenotype unconfirmed for human disease.

Show evidence (1 reference)

PMID:33268356 SUPPORT Model Organism

"A dominant zebrafish model (p.D123N) derived from a forward genetic screen has been previously reported with craniofacial abnormalities, which were replicated in our patient cohort"

Confirms that the craniofacial model evidence comes from the h3f3a paralog, which is the substance of the translational mismatch.

⚙

Pathophysiology

8

De Novo H3-3B Variant Altering Histone H3.3

BLBS2 begins with a de novo heterozygous variant in H3-3B, one of the two genes encoding the replication-independent histone variant H3.3. H3-3A and H3-3B produce an identical 135-amino-acid protein but differ in their regulatory and coding sequences, giving different expression patterns and levels — the only basis on which the two BLBS entities are separated. Unlike the recurrent oncohistone substitutions of pediatric glioma, BLBS alleles are largely private and distributed across the whole protein (tail and core), and include missense, synonymous/transcript-dependent and stop-loss changes. They act through gain-of-function or dominant-negative mechanisms, not haploinsufficiency, which is why a single altered allele of a highly constrained histone gene produces disease.

H3-3B hgnc:4765

structural constituent of chromatin GO:0030527

Downstream

Disrupted Nucleosome Assembly and Chaperone Interaction The altered residue sits at an interface with DNA, a partner histone, or an H3.3-specific chaperone, so the first consequence is a defect in how H3.3 is handled and incorporated into the nucleosome.

Show evidence (4 references)

PMID:34876591 SUPPORT Human Clinical

"The histone H3 variant H3.3, encoded by two genes H3-3A and H3-3B, can replace canonical isoforms H3.1 and H3.2."

Establishes that H3-3B encodes the replication-independent H3.3 variant that substitutes for the canonical replication-coupled H3 isoforms.

PMID:34876591 SUPPORT Human Clinical

"H3.3 is important in chromatin compaction, early embryonic development, and lineage commitment."

Gives the normal functions of H3.3 that are disrupted, linking the molecular lesion to a developmental phenotype.

PMID:36867246 SUPPORT Human Clinical

"Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."

States the allelic architecture (private, protein-wide) and the gain-of-function/dominant-negative direction of effect that defines this disorder's mechanism.

+ 1 more reference

Disrupted Nucleosome Assembly and Chaperone Interaction

Molecular modelling of the full BLBS variant set shows that the substituted residues disrupt contacts with DNA, with the other core histones, and with histone chaperones. Residues essential for H3.3-specific recognition (for example p.G90, required for DAXX/UBN1 binding, and p.S31, required for ZMYND11 recognition) are mutated in patients, and some alleles change how much H3.3 protein is present or how tightly it binds its chaperone: several constructs reduce H3.3 protein levels, one (p.R41C) accumulates above wild-type, and p.R129H binds the chaperone DAXX more strongly than wild-type. The common consequence is that the formation, deposition and stability of H3.3-containing nucleosomes is altered.

nucleosome assembly GO:0006334 ⚠ ABNORMAL

histone binding GO:0042393 ⚠ ABNORMAL DNA binding GO:0003677 ⚠ ABNORMAL

Downstream

Altered Chromatin Accessibility and Transcriptional Dysregulation Abnormal H3.3 deposition changes the chromatin landscape at H3.3-enriched regulatory regions and gene bodies, changing which genes are accessible and transcribed.

Aberrant Local Histone Post-Translational Modification Altered nucleosomal H3.3 changes the local substrate available to histone-modifying enzymes, shifting the PTM pattern in cis.

Show evidence (3 references)

PMID:33268356 SUPPORT Computational

"Molecular modeling of all 37 variants demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins."

Structural modelling across the whole variant set establishes disrupted DNA/histone/chaperone interaction as the shared proximal molecular defect.

PMID:34876591 SUPPORT In Vitro

"Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."

Functional evidence that BLBS alleles change H3.3 protein abundance in both directions, consistent with dysregulated deposition rather than simple loss.

PMID:34876591 SUPPORT In Vitro

"One H3.3 variant construct (p.R129H) was found to have stronger interaction with the chaperone death domain-associated protein 6."

Direct demonstration that a BLBS allele alters binding to the H3.3-specific chaperone DAXX.

Aberrant Local Histone Post-Translational Modification

Mass-spectrometric profiling of histones from BLBS patient cells — including lymphoblasts carrying the H3F3B p.A29P and p.P121R alleles — shows aberrant post-translational modification patterns that are local rather than global. This is a mechanistically important contrast with the somatic oncohistone lysine substitutions (H3.3 K27M, K36M), which poison methyltransferases in trans and cause genome-wide PTM collapse. In BLBS the epigenetic disturbance is restricted to the neighbourhood of the variant nucleosome, which helps explain why germline H3.3 variants cause a developmental syndrome without the malignancy risk of the oncohistones.

chromatin organization GO:0006325 ⚠ ABNORMAL

histone binding GO:0042393 ⚠ ABNORMAL

Downstream

Altered Chromatin Accessibility and Transcriptional Dysregulation Shifted local PTM patterns alter reader-protein recruitment and therefore the transcriptional output of the affected loci.

Show evidence (3 references)

PMID:33268356 SUPPORT In Vitro

"Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."

Establishes the local (not global) character of the PTM disturbance and explicitly separates it from the oncohistone mechanism.

PMID:33268356 SUPPORT In Vitro

"lymphoblasts (H3F3A p.A15G, H3F3A p.R17G, H3F3A p.T45I, H3F3B p.A29P, and H3F3B p.P121R) and primary fibroblasts"

Names the patient cell lines used for the PTM profiling; two of the five lymphoblast lines (H3F3B p.A29P and p.P121R) carry H3-3B alleles, giving BLBS2-specific support for the local-PTM finding.

PMID:34667098 SUPPORT Other

"Mutations in histone genes can affect sites of PTMs causing changes in local and global DNA methylation status."

Background review support for the general principle that histone-gene variants perturb PTM sites and downstream methylation state.

Altered Chromatin Accessibility and Transcriptional Dysregulation

The convergent cellular lesion is a remodelled chromatin landscape with coordinated transcriptional dysregulation. In an isogenic iPSC model of the H3-3B p.Leu48Arg allele, the variant increases H3-3B expression and drives hyper-deposition of H3.3 into nucleosomes; multi-omic profiling of the derived neural progenitors and forebrain neurons shows paired changes in chromatin accessibility and gene expression affecting neuronal fate, cell adhesion, neurotransmission and excitatory/inhibitory balance. In patient fibroblasts the transcriptional signature is dominated by up-regulated mitosis and cell-division genes.

neural progenitor cell CL:0011020 fibroblast CL:0000057

chromatin remodeling GO:0006338 ⚠ ABNORMAL regulation of transcription by RNA polymerase II GO:0006357 ⚠ ABNORMAL

Downstream

Impaired Neuronal Differentiation and Maturation Dysregulated fate and adhesion programs in neural progenitors translate into abnormal production and maturation of cortical neurons.

Neural Crest and Craniofacial Developmental Defect The same chromatin lesion perturbs the neural crest gene-regulatory program that builds the facial skeleton.

Deregulated Cell Proliferation Up-regulation of mitosis and cell-division genes increases proliferative capacity of patient cells.

Show evidence (3 references)

PMID:41689096 SUPPORT In Vitro

"In the 2D systems, we found dysregulated gene expression and chromatin accessibility affecting neuronal fate, adhesion, neurotransmission, and excitatory/inhibitory balance."

Human iPSC-derived neural cells carrying an H3-3B BLBS2 allele show the paired chromatin-accessibility and transcriptional changes that define this node.

PMID:41689096 SUPPORT In Vitro

"H3.3 L48R increases H3-3B expression, resulting in the hyper-deposition of H3.3 into the nucleosome, which underlies changes in gene expression and chromatin accessibility"

Identifies hyper-deposition of H3.3 into the nucleosome as the mechanism linking the H3-3B allele to the chromatin and transcriptional changes.

PMID:33268356 SUPPORT In Vitro

"RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."

Patient-derived cells show a reproducible transcriptional and proliferative consequence of the chromatin lesion.

Deregulated Cell Proliferation

Patient fibroblast lines carrying germline H3.3 variants — including the H3F3B p.G34V and H3F3B p.V117V/p.S146X alleles — proliferate faster than matched controls, matching the mitosis/cell-division transcriptional signature. This is proposed as the point of convergence between germline BLBS variants and the somatic oncohistone mutations. Importantly, the convergence is at the level of proliferative control only: germline carriers do not show the malignancy that follows somatic H3.3 substitution, and the community-wide cancer incidence in germline histonopathies (1%) falls below the threshold used to justify routine tumour surveillance.

fibroblast CL:0000057

regulation of cell population proliferation GO:0042127 ↑ INCREASED

Show evidence (3 references)

PMID:33268356 SUPPORT In Vitro

"Five H3F3A/B patient fibroblast lines (H3F3B: p.G34V; H3F3A: p.R17G; H3F3A: p.G90R; H3F3A: p.T45I; and H3F3B: p.V117V or p.S146X in alternate transcript) demonstrated increased proliferation over six matched controls."

Direct proliferation assay including two H3F3B (H3-3B) patient lines, giving BLBS2-specific support for the increased-proliferation node.

PMID:33268356 SUPPORT In Vitro

"These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."

States the authors' interpretation that germline and somatic H3.3 mechanisms differ in origin but converge on proliferative control.

PMID:40241305 SUPPORT Human Clinical

"the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"

Quantifies the absence of a clinically actionable cancer excess despite the increased proliferative capacity of patient cells.

Impaired Neuronal Differentiation and Maturation

In dorsal forebrain organoids carrying the H3-3B p.Leu48Arg allele the proportions of radial glia and mature neuronal populations are shifted, and whole-cell patch clamp shows that the neurons that do form are electrophysiologically immature, with decreased electrical activity relative to isogenic controls. This couples the chromatin lesion to a concrete neurodevelopmental deficit: fewer properly specified, properly maturing, properly firing cortical neurons.

radial glial cell CL:0000681 neuron CL:0000540

neuron differentiation GO:0030182 ⚠ ABNORMAL chemical synaptic transmission GO:0007268 ↓ DECREASED cell adhesion GO:0007155 ⚠ ABNORMAL

Downstream

Abnormal Brain Development and Progressive Neurologic Dysfunction Defective neuronal production, migration and maturation yields the structural brain malformations and the neurodevelopmental/neurodegenerative course.

Show evidence (3 references)

PMID:41689096 SUPPORT In Vitro

"Immunofluorescence of DFBOs corroborated altered proportions of radial glia and mature neuronal populations."

Shows that an H3-3B BLBS2 allele shifts the balance of neural progenitors and mature neurons in a human forebrain organoid.

PMID:41689096 SUPPORT In Vitro

"Patch clamp recordings revealed decreased electrical activity in neurons from L48R DFBOs compared to control DFBOs."

Functional electrophysiological deficit in BLBS2-allele neurons, linking the chromatin lesion to impaired neuronal maturation.

PMID:41689096 SUPPORT In Vitro

"Functionally, this causes dysregulation of cell adhesion, neurotransmission, and the balance between excitatory and inhibitory signaling."

Names the excitatory/inhibitory imbalance that plausibly underlies the epilepsy seen in BLBS2.

Neural Crest and Craniofacial Developmental Defect

H3.3 is required for the neural crest program that builds the craniofacial skeleton. In zebrafish, a dominant h3f3a p.D123N allele — two residues from a patient variant — reproduces craniofacial anomalies, and homozygous h3f3a mutants lose neural-crest-derived jaw cartilage entirely and show reduced foxd3-positive neural-crest-derived glia along with melanocyte and xanthophore loss. The glial defect is proposed to underlie the hypomyelination seen on patient neuroimaging, making this node a shared origin for the facial dysmorphism and part of the white-matter phenotype.

migratory neural crest cell CL:0000333 glial cell CL:0000125

neural crest cell differentiation GO:0014033 ⚠ ABNORMAL glial cell differentiation GO:0010001 ↓ DECREASED

Downstream

Abnormal Brain Development and Progressive Neurologic Dysfunction Loss of neural-crest-derived glia contributes to the hypomyelination component of the neuroimaging phenotype.

Show evidence (2 references)

PMID:33268356 SUPPORT Model Organism

"A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia."

Animal-model evidence tying H3.3 dysfunction to both the craniofacial and the glial arms of the phenotype.

PMID:33268356 SUPPORT Model Organism

"The loss of glial cells may relate to the hypomyelination phenotype that is noted on the brain MRIs of over one-third of the H3.3 cohort"

Links the neural-crest glial deficit in the model to the hypomyelination seen on patient imaging.

Abnormal Brain Development and Progressive Neurologic Dysfunction

The organism-level endpoint is a malformed, hypomyelinated brain with a characteristic imaging signature — small posterior fossa, dysgenesis of the corpus callosum (especially the splenium), cortical developmental abnormalities such as dysgyria and hypoplastic temporal poles, and deformity of the lateral ventricles — accompanied by developmental delay/intellectual disability, abnormal muscle tone, epilepsy and oculomotor dysfunction. Unlike a purely static encephalopathy, a subset of individuals show a progressive course with loss of milestones, worsening gait, spasticity and camptocormia, which is why BLBS was first described as a neurodegenerative disorder.

Show evidence (3 references)

PMID:35772801 SUPPORT Human Clinical

"Imaging phenotypes in germline H3F3-affected individuals are related to brain features, including a small posterior fossa as well as dysgenesis of the corpus callosum, cortical developmental abnormalities, and deformity of lateral ventricles."

Defines the structural brain phenotype that this endpoint node represents.

PMID:33268356 SUPPORT Human Clinical

"progressive neurologic dysfunction and congenital anomalies without malignancies"

Captures the progressive (neurodegenerative) character of the neurologic endpoint and the absence of tumour predisposition.

PMID:37782742 SUPPORT Human Clinical

"spasticity, loss of developmental milestones, worsening gait"

Enumerates the progressive neurologic features that distinguish BLBS from a static developmental encephalopathy.

⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

●

Phenotypes

42

Cardiovascular 1

Congenital Heart Defects OCCASIONAL Atrial septal defect HP:0001631

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Cardiac 11/82 (13%) Dermal 46/88 (52%)"

Quantifies cardiac anomalies at 11/82 (13%), within the OCCASIONAL band.

PMID:38678163 SUPPORT Human Clinical

"Fourteen percent of individuals present with cardiac anomalies,"

Confirms the cardiac-anomaly frequency in the narrative text.

Digestive 1

Constipation Constipation HP:0002019

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"monitor for constipation; assess for new manifestations, such as seizures and changes in tone and/or gait"

GeneReviews includes constipation monitoring in the routine surveillance schedule, implying it is a recurrent problem.

Ear 1

Hearing Impairment Hearing impairment HP:0000365

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"craniosynostosis, vision issues, hearing loss, congenital heart defects"

GeneReviews lists hearing loss among the manifestations requiring standard treatment.

Endocrine 1

Hypothyroidism Hypothyroidism HP:0000821

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"congenital heart defects, undescended testes, and hypothyroidism"

GeneReviews lists hypothyroidism among the manifestations requiring standard treatment, and recommends annual thyroid function tests.

Eye 2

Hypertelorism Hypertelorism HP:0000316

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"

Lists hypertelorism among the commonly reported craniofacial features.

Strabismus FREQUENT Strabismus HP:0000486

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Oculomotor 49/90 (54%) Strabismus 32/90 (36%)"

Quantifies strabismus at 32/90 (36%), within the FREQUENT band.

Genitourinary 1

Cryptorchidism Cryptorchidism HP:0000028

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"congenital heart defects, undescended testes, and hypothyroidism"

GeneReviews lists undescended testes among the manifestations requiring standard treatment.

Head and Neck 5

Craniofacial Dysmorphism VERY_FREQUENT Abnormal facial shape HP:0001999

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"92% of individuals with BLBS present with craniofacial anomalies"

Quantifies craniofacial anomalies at 86/93 (92%), within the VERY_FREQUENT band.

PMID:38678163 SUPPORT Human Clinical

"Commonly reported features are broad/prominent forehead,"

Names the specific dysmorphic features that make up the craniofacial phenotype.

Epicanthus Epicanthus HP:0000286

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"

Lists epicanthal folds among the commonly reported craniofacial features.

Craniosynostosis Craniosynostosis HP:0001363

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"craniosynostosis (of any suture), and ocular involvement"

GeneReviews includes craniosynostosis of any suture in the clinical description.

Microcephaly FREQUENT Microcephaly HP:0000252

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Microcephaly ( ≤5th percentile) 30/95 (32%)"

Quantifies microcephaly at 30/95 (32%), within the FREQUENT band.

Macrocephaly OCCASIONAL Macrocephaly HP:0000256

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Macrocephaly ( ≥95th percentile) 14/95 (15%)"

Quantifies macrocephaly at 14/95 (15%), within the OCCASIONAL band.

Integument 1

Dermatologic Anomalies OCCASIONAL Prominent fingertip pads HP:0001212

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Fetal ﬁnger pad 14/88 (16%)"

Quantifies fetal finger pads at 14/88 (16%), the specific dermatologic feature annotated here.

PMID:38678163 SUPPORT Human Clinical

"Dermal 46/88 (52%) Eczema 6/88 (7%) Nipple anomalies 15/88 (17%)"

Gives the overall dermatologic burden described in this phenotype entry.

Musculoskeletal 5

Generalized Hypotonia FREQUENT Generalized hypotonia HP:0001290

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Muscle tone anomalies Hypotonia 57/92 (62%) Hypertonia 11/92 (12%)"

Quantifies hypotonia at 57/92 (62%), within the FREQUENT band.

PMID:38678163 SUPPORT Human Clinical

"Eighty-four percent of individuals present with hypotonia, hypertonia, or a combination of axial hypotonia with peripheral hypertonia"

Gives the combined tone-abnormality frequency (84% with any tone abnormality) that contextualises the 62% hypotonia-specific figure above. The full clause is quoted deliberately: truncated at the first comma it would read as 84% having hypotonia, which contradicts the 57/92 denominator.

Hypertonia OCCASIONAL Hypertonia HP:0001276

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Hypotonia 57/92 (62%) Hypertonia 11/92 (12%) Axial hypotonia, peripheral hypertonia 9/92 (10%)"

Quantifies hypertonia at 11/92 (12%), within the OCCASIONAL band.

Spasticity Spasticity HP:0001257

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity"

GeneReviews lists spasticity among the neurologic findings of BLBS.

Scoliosis OCCASIONAL Scoliosis HP:0002650

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Musculoskeletal 56/94 (60%) Scoliosis 20/94 (21%)"

Quantifies scoliosis at 20/94 (21%), within the OCCASIONAL band.

Joint Hypermobility OCCASIONAL Joint hypermobility HP:0001382

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Lordosis/Kyphosis 4/94 (4%) Ligamentous laxity 21/94 (22%)"

Quantifies ligamentous laxity at 21/94 (22%), within the OCCASIONAL band.

Nervous System 9

Developmental Delay and Intellectual Disability VERY_FREQUENT Global developmental delay HP:0001263

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Developmental delay/intellectual disability 94/95 (99%)"

Quantifies the frequency (94/95, 99%) supporting the VERY_FREQUENT band.

PMID:37782742 SUPPORT Human Clinical

"characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities"

GeneReviews states that developmental delay/intellectual disability is a defining feature and typically severe.

Severe Intellectual Disability Severe intellectual disability HP:0010864

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Many affected individuals do not achieve independent sitting, walking, or speaking, although there is a range of developmental outcomes."

Describes the severity of the cognitive/motor outcome that supports the severe intellectual disability annotation.

Seizures FREQUENT Seizure HP:0001250

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Seizures were more frequently reported in individuals harboring variants in H3-3B (59%) than those harboring variants in H3-3A (45%)."

Gene-specific frequency: 59% in H3-3B (BLBS2) carriers, within the FREQUENT band and higher than in BLBS1.

PMID:38678163 SUPPORT Human Clinical

"Developmental delay/intellectual disability 94/95 (99%) Seizures 45/91 (49%)"

Cohort-wide denominator for seizures (45/91, 49%) in the combined BLBS series.

Developmental Regression Developmental regression HP:0002376

Show evidence (2 references)

PMID:37782742 SUPPORT Human Clinical

"spasticity, loss of developmental milestones, worsening gait"

GeneReviews lists loss of developmental milestones among the neurologic findings.

PMID:38678163 PARTIAL Human Clinical

"BLBS was initially categorized as a progressive neurodegenerative syndrome caused by de novo heterozygous variants in either H3-3A or H3-3B"

Supports the progressive framing but is a historical statement about the syndrome's classification rather than a frequency estimate, hence PARTIAL.

Structural Brain Abnormalities on Neuroimaging FREQUENT Abnormal cerebral morphology HP:0002060

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Fifty-eight percent of individuals with a reported MRI were diagnosed with at least one abnormal"

Anchors the neuroimaging-findings row of the cohort phenotype table (44/76, 58% of imaged individuals had at least one abnormal finding).

PMID:34876591 SUPPORT Human Clinical

"showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"

Independent confirmation that structural brain abnormalities are a major phenotype in H3-3A/H3-3B variant carriers.

Delayed Myelination Delayed myelination HP:0012448

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"included delayed myelination or hypomyelination; dysgenesis of the corpus collosum; dilated ventricles; and hemispheric asymmetry across multiple structures"

Lists delayed myelination/hypomyelination among the common MRI findings.

Ventriculomegaly OCCASIONAL Ventriculomegaly HP:0002119

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Corpus collosum malformation/dysgenesis 28/76 (37%) Dilated ventricles 6/76 (8%) Asymmetry 4/76 (5%)"

Quantifies ventricular dilatation at 6/76 (8%), within the OCCASIONAL band.

PMID:35772801 SUPPORT Human Clinical

"Six individuals (33%) presented with reduced size and an internal rotational appearance of the heads of the caudate nuclei along with an enlarged and squared appearance of the frontal horns of the lateral ventricles."

Describes the distinctive caudate/frontal-horn configuration reported in the imaging series.

Autism Spectrum Disorder OCCASIONAL Autism HP:0000717

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety"

GeneReviews lists autism spectrum disorder among the manifestations requiring standard treatment.

Delayed Speech and Language Development FREQUENT Delayed speech and language development HP:0000750

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Speaks at least one word (>20 months) 50/84 (60%)"

The complement of this figure (40% not speaking at all beyond 20 months, with further delay among those who do speak) supports frequent speech delay.

Growth 2

Short Stature FREQUENT Short stature HP:0004322

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Height ( ≤5th percentile) 32/91 (35%)"

Quantifies short stature at 32/91 (35%), within the FREQUENT band.

PMID:34876591 SUPPORT Human Clinical

"showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"

Independent cohort (including six H3-3B individuals) listing short stature among the major phenotypes.

Failure to Thrive and Feeding Difficulties Failure to thrive HP:0001508

Show evidence (2 references)

PMID:34876591 SUPPORT Human Clinical

"showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"

Lists failure to thrive among the major phenotypes in a cohort that included six H3-3B individuals.

PMID:37782742 SUPPORT Human Clinical

"Feeding issues and poor weight gain may benefit from feeding therapy"

Confirms feeding issues and poor weight gain as recognised manifestations requiring intervention.

Other 13

Axial Hypotonia with Peripheral Hypertonia OCCASIONAL Axial hypotonia HP:0008936

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Axial hypotonia, peripheral hypertonia 9/92 (10%)"

Quantifies the mixed tone pattern at 9/92 (10%).

Camptocormia Camptocormia HP:0100595

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"of developmental milestones, worsening gait, and/or camptocormia"

GeneReviews names camptocormia as one of the progressive neurologic findings that can occur in BLBS.

Paroxysmal Dyskinesia VERY_RARE Paroxysmal dyskinesia HP:0007166

Show evidence (1 reference)

PMID:40696776 PARTIAL Human Clinical

"Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene."

A single-patient clinical report linking paroxysmal dyskinesia to an H3-3B variant. The publication is a brief case letter without an abstract, so only the title assertion is quotable and the evidence is marked PARTIAL.

Broad Forehead Broad forehead HP:0000337

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Commonly reported features are broad/prominent forehead,"

Names broad/prominent forehead as a commonly reported feature.

Wide Nasal Bridge Wide nasal bridge HP:0000431

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"

Lists a broad nasal bridge among the commonly reported craniofacial features.

Thin Upper Lip Vermilion Thin upper lip vermilion HP:0000219

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"

Lists a thin upper lip among the commonly reported craniofacial features.

Small Posterior Fossa FREQUENT Small posterior fossa HP:0040010

Show evidence (1 reference)

PMID:35772801 SUPPORT Human Clinical

"Thirteen of 18 individuals (72%) presented with a small posterior fossa."

Quantifies small posterior fossa at 13/18 (72%) in the imaging cohort.

Corpus Callosum Dysgenesis FREQUENT Aplasia/Hypoplasia of the corpus callosum HP:0007370

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Corpus collosum malformation/dysgenesis 28/76 (37%)"

Quantifies corpus callosum malformation at 28/76 (37%), within the FREQUENT band.

PMID:35772801 SUPPORT Human Clinical

"Five individuals (28%) presented with dysgenesis of the splenium of the corpus callosum."

Localises the callosal defect to the splenium in the imaging series.

Cortical Developmental Abnormalities FREQUENT Abnormal cerebral cortex morphology HP:0002538

Show evidence (1 reference)

PMID:35772801 SUPPORT Human Clinical

"Cortical developmental abnormalities were noted in 8 individuals (44%), with dysgyria and hypoplastic temporal poles being the most frequent presentation."

Quantifies cortical developmental abnormalities at 8/18 (44%) and names the specific patterns.

Oculomotor Dysfunction FREQUENT Abnormal eye physiology HP:0012373

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Oculomotor 49/90 (54%) Strabismus 32/90 (36%) Astigmatism 7/88 (8%)"

Quantifies oculomotor dysfunction at 49/90 (54%), within the FREQUENT band.

Astigmatism OCCASIONAL Astigmatism HP:0000483

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"Strabismus 32/90 (36%) Astigmatism 7/88 (8%)"

Quantifies astigmatism at 7/88 (8%), within the OCCASIONAL band.

Genital Anomalies OCCASIONAL Abnormality of the genital system HP:0000078

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"duals with variants in H3-3B (28%) present with genital anomalies"

Gene-specific frequency showing genital anomalies are enriched in H3-3B (BLBS2) carriers.

PMID:37782742 SUPPORT Human Clinical

"Congenital anomalies are rare but can include congenital heart defects, brain malformations, and genitourinary abnormalities in males."

GeneReviews confirms genitourinary abnormalities in males as part of the congenital-anomaly spectrum.

Delayed Ability to Walk FREQUENT Delayed ability to walk HP:0031936

Show evidence (2 references)

PMID:38678163 SUPPORT Human Clinical

"Delayed/No walking (>20 months) 59/75 (79%)"

Quantifies delayed or unachieved walking at 59/75 (79%), within the FREQUENT band.

PMID:38678163 SUPPORT Human Clinical

"independent walking when compared to individuals harboring variants in H3-3B (65%)"

Gives the H3-3B-specific (BLBS2) figure for delayed or unachieved independent walking.

🧬

Genetic Associations

1

H3-3B (De novo heterozygous pathogenic variants)

Gene: H3-3B hgnc:4765 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (5 references)

PMID:38678163 SUPPORT Human Clinical

"Bryant-Li-Bhoj syndrome (BLBS), which became OMIM-classified in 2022 (OMIM: 619720, 619721), is caused by germline variants in the two genes that encode histone H3.3 (H3-3A/H3F3A and H3-3B/H3F3B)"

Establishes H3-3B as one of the two BLBS genes and ties it to the OMIM entities, of which 619721 is BLBS2.

PMID:34876591 SUPPORT Human Clinical

"Here we report eleven de novo missense variants and one de novo stop-loss variant in H3-3A (n = 6) and H3-3B (n = 6)"

Documents six independent de novo H3-3B variants, giving direct BLBS2-specific genetic evidence.

PMID:33268356 SUPPORT Human Clinical

"the gnomAD missense Z score is 3.21 for H3F3A and 2.95 for H3F3B (>2 is significant)"

Establishes strong missense constraint on H3F3B, supporting pathogenicity of de novo missense alleles in this gene.

+ 2 more references

💊

Medical Actions

6

Developmental and Rehabilitative Therapy

Action: Rehabilitation NCIT:C15315

There is no disease-modifying therapy for BLBS2. Management is supportive and follows standard practice for developmental delay/intellectual disability, with early intervention, physical, occupational and speech therapy directed at motor and communication milestones.

Target Phenotypes: Global developmental delay HP:0001263

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Standard treatment for developmental delay / intellectual disability"

GeneReviews recommends standard developmental management; no targeted therapy exists.

Feeding Therapy and Gastrostomy

Action: Supportive Care NCIT:C15747

Feeding issues and poor weight gain are managed with feeding therapy; a gastrostomy tube may be considered when feeding problems persist.

Target Phenotypes: Failure to thrive HP:0001508

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Feeding issues and poor weight gain may benefit from feeding therapy; in individuals with persistent feeding issues, gastrostomy tube may be considered."

Directly states the feeding-management recommendation.

Antiseizure Pharmacotherapy

Action: Pharmacotherapy NCIT:C15986

Epilepsy affects a majority of individuals with H3-3B variants and is managed with standard antiseizure medication; no BLBS-specific regimen or contraindication has been established.

Target Phenotypes: Seizure HP:0001250

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation"

GeneReviews directs standard treatment for epilepsy among the manifestations of BLBS. The snippet begins at the recommendation clause so it reads as a management statement rather than a list of features.

Craniosynostosis Surgery

Action: Surgical Procedure NCIT:C15329

Craniosynostosis of any suture may occur and is managed with standard surgical correction.

Target Phenotypes: Craniosynostosis HP:0001363

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation, craniosynostosis, vision issues"

GeneReviews lists craniosynostosis among the manifestations receiving standard treatment, which is surgical. The snippet begins at the recommendation clause so it reads as a management statement rather than a list of features.

Multisystem Surveillance

Action: Supportive Care NCIT:C15747

Recommended surveillance includes, at each visit, growth parameters, nutrition and oral intake safety, constipation, new manifestations such as seizures and changes in tone or gait, developmental progress and behaviour; and annually or as indicated, ophthalmology evaluation, audiology evaluation and thyroid function tests. Routine tumour surveillance is NOT recommended: the community-wide cancer incidence across germline histonopathies is currently 1%, below the 5% threshold used to justify a surveillance protocol.

Show evidence (2 references)

PMID:37782742 SUPPORT Human Clinical

"Annually or as clinically indicated: ophthalmology evaluation, audiology evaluation, and thyroid function tests."

Specifies the annual surveillance schedule.

PMID:40241305 SUPPORT Human Clinical

"the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"

Evidence for the explicit recommendation against routine tumour surveillance.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Counselling covers the autosomal dominant, typically de novo origin, the consequently low sibling recurrence risk, and the availability of prenatal and preimplantation genetic testing once the familial H3-3B variant is known.

Show evidence (1 reference)

PMID:37782742 SUPPORT Human Clinical

"Once an H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible."

States the reproductive options that genetic counselling covers.

🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Bryant-Li-Bhoj Neurodevelopmental Syndrome 2:

Bryant-Li-Bhoj neurodevelopmental syndrome 1 Not Yet Curated MONDO:0030606

Overlapping Features The H3-3A-related counterpart of this disorder. The two are clinically indistinguishable at the level of an individual patient — the protein product is identical — and are separated only by which paralog carries the variant. Cohort-level differences favour BLBS1 for delayed walking (85% vs 65%) and dysmorphism, and BLBS2 for seizures (59% vs 45%) and genital anomalies (28% vs 17%), but these are group trends, not diagnostic discriminators.

Distinguishing Features

Determined solely by molecular testing: a pathogenic variant in H3-3A rather than H3-3B
Cohort-level trend toward more frequent delayed walking and dysmorphism than in BLBS2
Cohort-level trend toward fewer seizures and fewer genital anomalies than in BLBS2

Show evidence (1 reference)

PMID:38678163 SUPPORT Human Clinical

"While specific genotype-phenotype correlations have not been conclusively delineated"

Confirms that no established genotype-phenotype rule separates the two entities clinically.

Diffuse midline glioma, H3 K27-altered

Overlapping Features The somatic counterpart of H3.3 disease. Recurrent somatic H3.3 substitutions (K27M, G34R/V) in glial precursors drive pediatric high-grade glioma with genome-wide collapse of histone post-translational modifications. Germline BLBS variants are private, scattered, cause only local PTM changes, and are not associated with malignancy — the 46-patient founding cohort had congenital anomalies without tumours.

Distinguishing Features

Somatic and tumour-restricted rather than germline and constitutional
Recurrent hotspot substitutions (K27M, G34R/V) rather than private protein-wide variants
Global histone PTM dysregulation rather than local in-cis PTM change
Malignancy as the presenting problem rather than a congenital neurodevelopmental syndrome

Show evidence (1 reference)

PMID:33268356 SUPPORT Human Clinical

"Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."

Contrasts the germline (local) and somatic (global) epigenetic mechanisms that separate BLBS from the oncohistone diseases.

🧫

Experimental Models

1

H3-3B p.Leu48Arg iPSC-derived neural model IPSC_DERIVED_MODEL

Induced pluripotent stem cells carrying the BLBS2 allele H3-3B p.Leu48Arg, differentiated into 2D neural progenitor cells, 2D forebrain neurons and 3D dorsal forebrain organoids, with paired transcriptomic and chromatin-accessibility profiling, immunofluorescence cell-identity mapping and whole-cell patch clamp. This is the first human-derived mechanistic model of BLBS and the principal BLBS2-specific model system available.

Organism

human NCBITaxon:9606

Publication

PMID:41689096

Findings

Paired chromatin-accessibility and transcriptomic changes affect neuronal fate, cell adhesion, neurotransmission and excitatory/inhibitory balance.

"In the 2D systems, we found dysregulated gene expression and chromatin accessibility affecting neuronal fate, adhesion, neurotransmission, and excitatory/inhibitory balance."

Organoids show altered proportions of radial glia and mature neurons, and the neurons that form are electrophysiologically immature.

"Patch clamp recordings revealed decreased electrical activity in neurons from L48R DFBOs compared to control DFBOs."

The proposed proximal mechanism is increased H3-3B expression driving hyper-deposition of H3.3 into the nucleosome.

"H3.3 L48R increases H3-3B expression, resulting in the hyper-deposition of H3.3 into the nucleosome, which underlies changes in gene expression and chromatin accessibility"

Show evidence (1 reference)

PMID:41689096 SUPPORT In Vitro

"These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"

Establishes the model's standing as the first human-derived mechanistic system for BLBS.

{ }

Source YAML

click to show

name: Bryant-Li-Bhoj Neurodevelopmental Syndrome 2
creation_date: "2026-08-01T00:00:00Z"
description: >-
  Bryant-Li-Bhoj neurodevelopmental syndrome 2 (BLBS2, BRYLIB2) is an autosomal
  dominant neurodevelopmental and neurodegenerative disorder caused by de novo
  heterozygous variants in H3-3B (H3F3B). It is one of the very few Mendelian
  diseases caused by variants in a histone protein itself rather than in a
  histone-modifying enzyme or chaperone — a germline histonopathy rather than a
  chromatin-regulator chromatinopathy. H3-3B and its paralog H3-3A encode an
  identical protein, the replication-independent histone variant H3.3, but differ
  in their untranslated and coding sequences and therefore in expression pattern
  and level; this paralog-level difference, not a difference in protein product,
  is why BLBS is split into two OMIM entities (BLBS1 for H3-3A, BLBS2 for H3-3B).
  Nearly all affected individuals have developmental delay/intellectual
  disability, and most have craniofacial dysmorphism, abnormal muscle tone and
  abnormal neuroimaging; seizures are reported more often with H3-3B than with
  H3-3A variants. The causative alleles are private, scattered across the whole
  protein, and act by gain-of-function or dominant-negative mechanisms rather
  than haploinsufficiency, which distinguishes them from the recurrent
  oncohistone substitutions (K27M, G34R/V) that drive pediatric glioma.
category: Mendelian
parents:
- syndromic disease
- neurodevelopmental disorder
- hereditary disease
synonyms:
- BLBS2
- BRYLIB2
- Bryant-Li-Bhoj syndrome 2
- H3-3B-related neurodevelopmental syndrome
- H3F3B-related neurodevelopmental disorder
disease_term:
  preferred_term: Bryant-Li-Bhoj neurodevelopmental syndrome 2
  term:
    id: MONDO:0030607
    label: Bryant-Li-Bhoj neurodevelopmental syndrome 2
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      A Mendelian disorder of a core chromatin structural protein; the entry is
      organised around the genotype-to-chromatin mechanism.
  - classification_value: NEUROLOGIC
    notes: >-
      The dominant clinical burden is neurodevelopmental and neurodegenerative
      (developmental delay/intellectual disability, epilepsy, tone abnormality,
      structural brain malformation).
inheritance:
- name: Autosomal dominant inheritance
  description: >-
    BLBS2 is inherited in an autosomal dominant manner and is typically caused by
    a de novo heterozygous H3-3B variant, so recurrence risk to siblings is low.
    No biallelic or recessive form has been described; the disease alleles are
    missense, synonymous (transcript-dependent) and stop-loss changes acting
    through gain-of-function or dominant-negative effects rather than loss of one
    copy.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "BRYLIB is expressed in an autosomal dominant manner and typically caused by a de novo H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant."
    explanation: >-
      GeneReviews assigns the disorder to autosomal dominant inheritance with a
      predominantly de novo mutational origin.
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Therefore, the risk to other family members is presumed to be low."
    explanation: >-
      States the low familial recurrence risk that follows from the de novo origin,
      which is what families are counselled on.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We analyze 46 patients bearing de novo germline mutations in histone 3 family 3A (H3F3A) or H3F3B with progressive neurologic dysfunction and congenital anomalies without malignancies."
    explanation: >-
      The founding cohort establishes the de novo germline origin of the causative
      H3F3B variants.
pathophysiology:
- name: De Novo H3-3B Variant Altering Histone H3.3
  biological_scale: MOLECULAR
  description: >-
    BLBS2 begins with a de novo heterozygous variant in H3-3B, one of the two
    genes encoding the replication-independent histone variant H3.3. H3-3A and
    H3-3B produce an identical 135-amino-acid protein but differ in their
    regulatory and coding sequences, giving different expression patterns and
    levels — the only basis on which the two BLBS entities are separated. Unlike
    the recurrent oncohistone substitutions of pediatric glioma, BLBS alleles are
    largely private and distributed across the whole protein (tail and core), and
    include missense, synonymous/transcript-dependent and stop-loss changes. They
    act through gain-of-function or dominant-negative mechanisms, not
    haploinsufficiency, which is why a single altered allele of a highly
    constrained histone gene produces disease.
  genes:
  - preferred_term: H3-3B
    term:
      id: hgnc:4765
      label: H3-3B
  molecular_functions:
  - preferred_term: structural constituent of chromatin
    term:
      id: GO:0030527
      label: structural constituent of chromatin
  evidence:
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The histone H3 variant H3.3, encoded by two genes H3-3A and H3-3B, can replace canonical isoforms H3.1 and H3.2."
    explanation: >-
      Establishes that H3-3B encodes the replication-independent H3.3 variant that
      substitutes for the canonical replication-coupled H3 isoforms.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "H3.3 is important in chromatin compaction, early embryonic development, and lineage commitment."
    explanation: >-
      Gives the normal functions of H3.3 that are disrupted, linking the molecular
      lesion to a developmental phenotype.
  - reference: PMID:36867246
    reference_title: "Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."
    explanation: >-
      States the allelic architecture (private, protein-wide) and the
      gain-of-function/dominant-negative direction of effect that defines this
      disorder's mechanism.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this larger cohort of 96 people, we identify causative missense, synonymous, and stop-loss variants."
    explanation: >-
      Documents the three variant classes seen in BLBS, beyond simple missense.
  downstream:
  - target: Disrupted Nucleosome Assembly and Chaperone Interaction
    description: >-
      The altered residue sits at an interface with DNA, a partner histone, or an
      H3.3-specific chaperone, so the first consequence is a defect in how H3.3 is
      handled and incorporated into the nucleosome.
- name: Disrupted Nucleosome Assembly and Chaperone Interaction
  biological_scale: MOLECULAR
  description: >-
    Molecular modelling of the full BLBS variant set shows that the substituted
    residues disrupt contacts with DNA, with the other core histones, and with
    histone chaperones. Residues essential for H3.3-specific recognition (for
    example p.G90, required for DAXX/UBN1 binding, and p.S31, required for
    ZMYND11 recognition) are mutated in patients, and some alleles change how much
    H3.3 protein is present or how tightly it binds its chaperone: several
    constructs reduce H3.3 protein levels, one (p.R41C) accumulates above
    wild-type, and p.R129H binds the chaperone DAXX more strongly than wild-type.
    The common consequence is that the formation, deposition and stability of
    H3.3-containing nucleosomes is altered.
  molecular_functions:
  - preferred_term: histone binding
    term:
      id: GO:0042393
      label: histone binding
    modifier: ABNORMAL
  - preferred_term: DNA binding
    term:
      id: GO:0003677
      label: DNA binding
    modifier: ABNORMAL
  biological_processes:
  - preferred_term: nucleosome assembly
    term:
      id: GO:0006334
      label: nucleosome assembly
    modifier: ABNORMAL
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Molecular modeling of all 37 variants demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins."
    explanation: >-
      Structural modelling across the whole variant set establishes disrupted
      DNA/histone/chaperone interaction as the shared proximal molecular defect.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."
    explanation: >-
      Functional evidence that BLBS alleles change H3.3 protein abundance in both
      directions, consistent with dysregulated deposition rather than simple loss.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "One H3.3 variant construct (p.R129H) was found to have stronger interaction with the chaperone death domain-associated protein 6."
    explanation: >-
      Direct demonstration that a BLBS allele alters binding to the H3.3-specific
      chaperone DAXX.
  downstream:
  - target: Altered Chromatin Accessibility and Transcriptional Dysregulation
    description: >-
      Abnormal H3.3 deposition changes the chromatin landscape at H3.3-enriched
      regulatory regions and gene bodies, changing which genes are accessible and
      transcribed.
  - target: Aberrant Local Histone Post-Translational Modification
    description: >-
      Altered nucleosomal H3.3 changes the local substrate available to
      histone-modifying enzymes, shifting the PTM pattern in cis.
- name: Aberrant Local Histone Post-Translational Modification
  biological_scale: MOLECULAR
  description: >-
    Mass-spectrometric profiling of histones from BLBS patient cells — including
    lymphoblasts carrying the H3F3B p.A29P and p.P121R alleles — shows aberrant
    post-translational modification patterns that are local rather than global.
    This is a mechanistically important contrast with the somatic oncohistone
    lysine substitutions (H3.3 K27M, K36M), which poison methyltransferases in
    trans and cause genome-wide PTM collapse. In BLBS the epigenetic disturbance
    is restricted to the neighbourhood of the variant nucleosome, which helps
    explain why germline H3.3 variants cause a developmental syndrome without the
    malignancy risk of the oncohistones.
  molecular_functions:
  - preferred_term: histone binding
    term:
      id: GO:0042393
      label: histone binding
    modifier: ABNORMAL
  biological_processes:
  - preferred_term: chromatin organization
    term:
      id: GO:0006325
      label: chromatin organization
    modifier: ABNORMAL
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
    explanation: >-
      Establishes the local (not global) character of the PTM disturbance and
      explicitly separates it from the oncohistone mechanism.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "lymphoblasts (H3F3A p.A15G, H3F3A p.R17G, H3F3A p.T45I, H3F3B p.A29P, and H3F3B p.P121R) and primary fibroblasts"
    explanation: >-
      Names the patient cell lines used for the PTM profiling; two of the five
      lymphoblast lines (H3F3B p.A29P and p.P121R) carry H3-3B alleles, giving
      BLBS2-specific support for the local-PTM finding.
  - reference: PMID:34667098
    reference_title: "Genes of the month: H3.3 histone genes: H3F3A and H3F3B"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Mutations in histone genes can affect sites of PTMs causing changes in local and global DNA methylation status."
    explanation: >-
      Background review support for the general principle that histone-gene variants
      perturb PTM sites and downstream methylation state.
  downstream:
  - target: Altered Chromatin Accessibility and Transcriptional Dysregulation
    description: >-
      Shifted local PTM patterns alter reader-protein recruitment and therefore the
      transcriptional output of the affected loci.
- name: Altered Chromatin Accessibility and Transcriptional Dysregulation
  biological_scale: CELLULAR
  description: >-
    The convergent cellular lesion is a remodelled chromatin landscape with
    coordinated transcriptional dysregulation. In an isogenic iPSC model of the
    H3-3B p.Leu48Arg allele, the variant increases H3-3B expression and drives
    hyper-deposition of H3.3 into nucleosomes; multi-omic profiling of the derived
    neural progenitors and forebrain neurons shows paired changes in chromatin
    accessibility and gene expression affecting neuronal fate, cell adhesion,
    neurotransmission and excitatory/inhibitory balance. In patient fibroblasts
    the transcriptional signature is dominated by up-regulated mitosis and
    cell-division genes.
  biological_processes:
  - preferred_term: chromatin remodeling
    term:
      id: GO:0006338
      label: chromatin remodeling
    modifier: ABNORMAL
  - preferred_term: regulation of transcription by RNA polymerase II
    term:
      id: GO:0006357
      label: regulation of transcription by RNA polymerase II
    modifier: ABNORMAL
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In the 2D systems, we found dysregulated gene expression and chromatin accessibility affecting neuronal fate, adhesion, neurotransmission, and excitatory/inhibitory balance."
    explanation: >-
      Human iPSC-derived neural cells carrying an H3-3B BLBS2 allele show the paired
      chromatin-accessibility and transcriptional changes that define this node.
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "H3.3 L48R increases H3-3B expression, resulting in the hyper-deposition of H3.3 into the nucleosome, which underlies changes in gene expression and chromatin accessibility"
    explanation: >-
      Identifies hyper-deposition of H3.3 into the nucleosome as the mechanism
      linking the H3-3B allele to the chromatin and transcriptional changes.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."
    explanation: >-
      Patient-derived cells show a reproducible transcriptional and proliferative
      consequence of the chromatin lesion.
  downstream:
  - target: Impaired Neuronal Differentiation and Maturation
    description: >-
      Dysregulated fate and adhesion programs in neural progenitors translate into
      abnormal production and maturation of cortical neurons.
  - target: Neural Crest and Craniofacial Developmental Defect
    description: >-
      The same chromatin lesion perturbs the neural crest gene-regulatory program
      that builds the facial skeleton.
  - target: Deregulated Cell Proliferation
    description: >-
      Up-regulation of mitosis and cell-division genes increases proliferative
      capacity of patient cells.
- name: Deregulated Cell Proliferation
  biological_scale: CELLULAR
  description: >-
    Patient fibroblast lines carrying germline H3.3 variants — including the H3F3B
    p.G34V and H3F3B p.V117V/p.S146X alleles — proliferate faster than matched
    controls, matching the mitosis/cell-division transcriptional signature. This is
    proposed as the point of convergence between germline BLBS variants and the
    somatic oncohistone mutations. Importantly, the convergence is at the level of
    proliferative control only: germline carriers do not show the malignancy that
    follows somatic H3.3 substitution, and the community-wide cancer incidence in
    germline histonopathies (1%) falls below the threshold used to justify routine
    tumour surveillance.
  biological_processes:
  - preferred_term: regulation of cell population proliferation
    term:
      id: GO:0042127
      label: regulation of cell population proliferation
    modifier: INCREASED
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Five H3F3A/B patient fibroblast lines (H3F3B: p.G34V; H3F3A: p.R17G; H3F3A: p.G90R; H3F3A: p.T45I; and H3F3B: p.V117V or p.S146X in alternate transcript) demonstrated increased proliferation over six matched controls."
    explanation: >-
      Direct proliferation assay including two H3F3B (H3-3B) patient lines, giving
      BLBS2-specific support for the increased-proliferation node.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."
    explanation: >-
      States the authors' interpretation that germline and somatic H3.3 mechanisms
      differ in origin but converge on proliferative control.
  - reference: PMID:40241305
    reference_title: "Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"
    explanation: >-
      Quantifies the absence of a clinically actionable cancer excess despite the
      increased proliferative capacity of patient cells.
- name: Impaired Neuronal Differentiation and Maturation
  biological_scale: CELLULAR
  description: >-
    In dorsal forebrain organoids carrying the H3-3B p.Leu48Arg allele the
    proportions of radial glia and mature neuronal populations are shifted, and
    whole-cell patch clamp shows that the neurons that do form are
    electrophysiologically immature, with decreased electrical activity relative to
    isogenic controls. This couples the chromatin lesion to a concrete
    neurodevelopmental deficit: fewer properly specified, properly maturing,
    properly firing cortical neurons.
  cell_types:
  - preferred_term: radial glial cell
    term:
      id: CL:0000681
      label: radial glial cell
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: neuron differentiation
    term:
      id: GO:0030182
      label: neuron differentiation
    modifier: ABNORMAL
  - preferred_term: chemical synaptic transmission
    term:
      id: GO:0007268
      label: chemical synaptic transmission
    modifier: DECREASED
  - preferred_term: cell adhesion
    term:
      id: GO:0007155
      label: cell adhesion
    modifier: ABNORMAL
  evidence:
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Immunofluorescence of DFBOs corroborated altered proportions of radial glia and mature neuronal populations."
    explanation: >-
      Shows that an H3-3B BLBS2 allele shifts the balance of neural progenitors and
      mature neurons in a human forebrain organoid.
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Patch clamp recordings revealed decreased electrical activity in neurons from L48R DFBOs compared to control DFBOs."
    explanation: >-
      Functional electrophysiological deficit in BLBS2-allele neurons, linking the
      chromatin lesion to impaired neuronal maturation.
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functionally, this causes dysregulation of cell adhesion, neurotransmission, and the balance between excitatory and inhibitory signaling."
    explanation: >-
      Names the excitatory/inhibitory imbalance that plausibly underlies the epilepsy
      seen in BLBS2.
  downstream:
  - target: Abnormal Brain Development and Progressive Neurologic Dysfunction
    description: >-
      Defective neuronal production, migration and maturation yields the structural
      brain malformations and the neurodevelopmental/neurodegenerative course.
- name: Neural Crest and Craniofacial Developmental Defect
  biological_scale: TISSUE
  description: >-
    H3.3 is required for the neural crest program that builds the craniofacial
    skeleton. In zebrafish, a dominant h3f3a p.D123N allele — two residues from a
    patient variant — reproduces craniofacial anomalies, and homozygous h3f3a
    mutants lose neural-crest-derived jaw cartilage entirely and show reduced
    foxd3-positive neural-crest-derived glia along with melanocyte and xanthophore
    loss. The glial defect is proposed to underlie the hypomyelination seen on
    patient neuroimaging, making this node a shared origin for the facial
    dysmorphism and part of the white-matter phenotype.
  cell_types:
  - preferred_term: migratory neural crest cell
    term:
      id: CL:0000333
      label: migratory neural crest cell
  - preferred_term: glial cell
    term:
      id: CL:0000125
      label: glial cell
  biological_processes:
  - preferred_term: neural crest cell differentiation
    term:
      id: GO:0014033
      label: neural crest cell differentiation
    modifier: ABNORMAL
  - preferred_term: glial cell differentiation
    term:
      id: GO:0010001
      label: glial cell differentiation
    modifier: DECREASED
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia."
    explanation: >-
      Animal-model evidence tying H3.3 dysfunction to both the craniofacial and the
      glial arms of the phenotype.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The loss of glial cells may relate to the hypomyelination phenotype that is noted on the brain MRIs of over one-third of the H3.3 cohort"
    explanation: >-
      Links the neural-crest glial deficit in the model to the hypomyelination seen
      on patient imaging.
  downstream:
  - target: Abnormal Brain Development and Progressive Neurologic Dysfunction
    description: >-
      Loss of neural-crest-derived glia contributes to the hypomyelination component
      of the neuroimaging phenotype.
- name: Abnormal Brain Development and Progressive Neurologic Dysfunction
  biological_scale: ORGANISM
  description: >-
    The organism-level endpoint is a malformed, hypomyelinated brain with a
    characteristic imaging signature — small posterior fossa, dysgenesis of the
    corpus callosum (especially the splenium), cortical developmental abnormalities
    such as dysgyria and hypoplastic temporal poles, and deformity of the lateral
    ventricles — accompanied by developmental delay/intellectual disability,
    abnormal muscle tone, epilepsy and oculomotor dysfunction. Unlike a purely
    static encephalopathy, a subset of individuals show a progressive course with
    loss of milestones, worsening gait, spasticity and camptocormia, which is why
    BLBS was first described as a neurodegenerative disorder.
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Imaging phenotypes in germline H3F3-affected individuals are related to brain features, including a small posterior fossa as well as dysgenesis of the corpus callosum, cortical developmental abnormalities, and deformity of lateral ventricles."
    explanation: >-
      Defines the structural brain phenotype that this endpoint node represents.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "progressive neurologic dysfunction and congenital anomalies without malignancies"
    explanation: >-
      Captures the progressive (neurodegenerative) character of the neurologic
      endpoint and the absence of tumour predisposition.
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "spasticity, loss of developmental milestones, worsening gait"
    explanation: >-
      Enumerates the progressive neurologic features that distinguish BLBS from a
      static developmental encephalopathy.
mechanistic_hypotheses:
- hypothesis_group_id: h33_hyperdeposition_model
  hypothesis_label: H3.3 hyper-deposition model of BLBS2
  status: EMERGING
  description: >-
    Proposes that the H3-3B disease allele acts by increasing H3-3B expression and
    driving excess incorporation of H3.3 into nucleosomes, so that the primary
    lesion is too much variant histone in chromatin rather than a loss of histone
    function. Demonstrated so far for a single allele (p.Leu48Arg) in one isogenic
    iPSC series; whether it generalises to tail-domain, core-domain and stop-loss
    BLBS2 alleles is untested, and it sits alongside the alternative view that
    different variants act through heterogeneous local mechanisms (impaired
    chaperone binding, altered protein stability, disrupted DNA contact).
  evidence:
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"
    explanation: >-
      The hyper-deposition model rests on a single human-derived model system, which
      is why it is recorded as emerging rather than canonical.
phenotypes:
- category: Neurologic
  name: Developmental Delay and Intellectual Disability
  description: >-
    Essentially universal in BLBS. Ninety-nine percent of the 96-person combined
    BLBS cohort had developmental delay/intellectual disability, typically in the
    severe range. The single reported exception carried an H3-3B
    synonymous/stop-loss allele and still had delayed speech and abnormal
    neuroimaging.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Developmental delay/intellectual disability 94/95 (99%)"
    explanation: >-
      Quantifies the frequency (94/95, 99%) supporting the VERY_FREQUENT band.
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities"
    explanation: >-
      GeneReviews states that developmental delay/intellectual disability is a
      defining feature and typically severe.
- category: Neurologic
  name: Severe Intellectual Disability
  description: >-
    Where intellectual disability is graded, it is typically severe; many affected
    individuals never achieve independent sitting, walking or speech.
  phenotype_term:
    preferred_term: Severe intellectual disability
    term:
      id: HP:0010864
      label: Severe intellectual disability
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Many affected individuals do not achieve independent sitting, walking, or speaking, although there is a range of developmental outcomes."
    explanation: >-
      Describes the severity of the cognitive/motor outcome that supports the severe
      intellectual disability annotation.
- category: Neurologic
  name: Seizures
  description: >-
    Seizures occur in about half of individuals with BLBS overall and are reported
    more frequently with H3-3B variants (59%) than with H3-3A variants (45%),
    making epilepsy one of the few features that appears enriched in BLBS2
    specifically. Roughly one in five of those with seizures had febrile seizures.
    A 2025 pan-histonopathy deep-phenotyping analysis found weaker support for
    seizures as a consistent syndrome-defining feature, so the frequency band here
    should be read as cohort-derived rather than settled.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seizures were more frequently reported in individuals harboring variants in H3-3B (59%) than those harboring variants in H3-3A (45%)."
    explanation: >-
      Gene-specific frequency: 59% in H3-3B (BLBS2) carriers, within the FREQUENT
      band and higher than in BLBS1.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Developmental delay/intellectual disability 94/95 (99%) Seizures 45/91 (49%)"
    explanation: >-
      Cohort-wide denominator for seizures (45/91, 49%) in the combined BLBS series.
- category: Neurologic
  name: Generalized Hypotonia
  description: >-
    Hypotonia is the single most common tone abnormality, present in 62% of the
    combined BLBS cohort; 84% have some tone abnormality (hypotonia, hypertonia, or
    axial hypotonia with peripheral hypertonia).
  phenotype_term:
    preferred_term: Generalized hypotonia
    term:
      id: HP:0001290
      label: Generalized hypotonia
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle tone anomalies Hypotonia 57/92 (62%) Hypertonia 11/92 (12%)"
    explanation: >-
      Quantifies hypotonia at 57/92 (62%), within the FREQUENT band.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Eighty-four percent of individuals present with hypotonia, hypertonia, or a combination of axial hypotonia with peripheral hypertonia"
    explanation: >-
      Gives the combined tone-abnormality frequency (84% with any tone abnormality)
      that contextualises the 62% hypotonia-specific figure above. The full clause is
      quoted deliberately: truncated at the first comma it would read as 84% having
      hypotonia, which contradicts the 57/92 denominator.
- category: Neurologic
  name: Hypertonia
  description: >-
    Twelve percent of individuals present with hypertonia, and a further 10% have
    the distinctive combination of axial hypotonia with peripheral hypertonia (or
    dystonia), which in the combined cohort occurred exclusively in carriers of
    histone-core variants.
  phenotype_term:
    preferred_term: Hypertonia
    term:
      id: HP:0001276
      label: Hypertonia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hypotonia 57/92 (62%) Hypertonia 11/92 (12%) Axial hypotonia, peripheral hypertonia 9/92 (10%)"
    explanation: >-
      Quantifies hypertonia at 11/92 (12%), within the OCCASIONAL band.
- category: Neurologic
  name: Axial Hypotonia with Peripheral Hypertonia
  description: >-
    Ten percent of individuals show concurrent axial hypotonia and peripheral
    hypertonia or dystonia. In the combined cohort this pattern was seen only in
    carriers of variants located in the histone core.
  phenotype_term:
    preferred_term: Axial hypotonia
    term:
      id: HP:0008936
      label: Axial hypotonia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Axial hypotonia, peripheral hypertonia 9/92 (10%)"
    explanation: >-
      Quantifies the mixed tone pattern at 9/92 (10%).
- category: Neurologic
  name: Developmental Regression
  description: >-
    A subset of individuals lose previously acquired milestones and show worsening
    gait, the feature that led to BLBS being described initially as a progressive
    neurodegenerative syndrome.
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "spasticity, loss of developmental milestones, worsening gait"
    explanation: >-
      GeneReviews lists loss of developmental milestones among the neurologic
      findings.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "BLBS was initially categorized as a progressive neurodegenerative syndrome caused by de novo heterozygous variants in either H3-3A or H3-3B"
    explanation: >-
      Supports the progressive framing but is a historical statement about the
      syndrome's classification rather than a frequency estimate, hence PARTIAL.
- category: Neurologic
  name: Spasticity
  description: >-
    Spasticity is one of the progressive neurologic findings that may emerge over
    time.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity"
    explanation: >-
      GeneReviews lists spasticity among the neurologic findings of BLBS.
- category: Musculoskeletal
  name: Camptocormia
  description: >-
    Forward flexion of the spine when standing that resolves when lying down; a
    distinctive late/progressive feature listed in the GeneReviews clinical
    description.
  phenotype_term:
    preferred_term: Camptocormia
    term:
      id: HP:0100595
      label: Camptocormia
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "of developmental milestones, worsening gait, and/or camptocormia"
    explanation: >-
      GeneReviews names camptocormia as one of the progressive neurologic findings
      that can occur in BLBS.
- category: Neurologic
  name: Paroxysmal Dyskinesia
  description: >-
    A movement-disorder presentation reported in a single individual with a novel
    H3-3B variant; not part of the core phenotype but relevant to the emerging
    BLBS2 spectrum.
  phenotype_term:
    preferred_term: Paroxysmal dyskinesia
    term:
      id: HP:0007166
      label: Paroxysmal dyskinesia
  frequency: VERY_RARE
  evidence:
  - reference: PMID:40696776
    reference_title: "Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene"
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene."
    explanation: >-
      A single-patient clinical report linking paroxysmal dyskinesia to an H3-3B
      variant. The publication is a brief case letter without an abstract, so only
      the title assertion is quotable and the evidence is marked PARTIAL.
- category: Craniofacial
  name: Craniofacial Dysmorphism
  description: >-
    Nonspecific but near-universal craniofacial anomalies (86/93, 92% of the
    combined cohort), commonly a broad or prominent forehead, broad nasal bridge,
    thin upper lip, hypertelorism, epicanthal folds, and ear and tooth anomalies.
    In the gene-stratified analysis, dysmorphic features were slightly less common
    with H3-3B (86%) than H3-3A (95%) variants.
  phenotype_term:
    preferred_term: Abnormal facial shape
    term:
      id: HP:0001999
      label: Abnormal facial shape
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "92% of individuals with BLBS present with craniofacial anomalies"
    explanation: >-
      Quantifies craniofacial anomalies at 86/93 (92%), within the VERY_FREQUENT
      band.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Commonly reported features are broad/prominent forehead,"
    explanation: >-
      Names the specific dysmorphic features that make up the craniofacial phenotype.
- category: Craniofacial
  name: Hypertelorism
  description: Increased interocular distance, among the commonly reported facial features.
  phenotype_term:
    preferred_term: Hypertelorism
    term:
      id: HP:0000316
      label: Hypertelorism
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"
    explanation: >-
      Lists hypertelorism among the commonly reported craniofacial features.
- category: Craniofacial
  name: Broad Forehead
  description: A broad or prominent forehead is one of the commonly reported facial features.
  phenotype_term:
    preferred_term: Broad forehead
    term:
      id: HP:0000337
      label: Broad forehead
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Commonly reported features are broad/prominent forehead,"
    explanation: >-
      Names broad/prominent forehead as a commonly reported feature.
- category: Craniofacial
  name: Wide Nasal Bridge
  description: A broad nasal bridge is among the commonly reported facial features.
  phenotype_term:
    preferred_term: Wide nasal bridge
    term:
      id: HP:0000431
      label: Wide nasal bridge
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"
    explanation: >-
      Lists a broad nasal bridge among the commonly reported craniofacial features.
- category: Craniofacial
  name: Thin Upper Lip Vermilion
  description: A thin upper lip is among the commonly reported facial features.
  phenotype_term:
    preferred_term: Thin upper lip vermilion
    term:
      id: HP:0000219
      label: Thin upper lip vermilion
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"
    explanation: >-
      Lists a thin upper lip among the commonly reported craniofacial features.
- category: Craniofacial
  name: Epicanthus
  description: Epicanthal folds are among the commonly reported facial features.
  phenotype_term:
    preferred_term: Epicanthus
    term:
      id: HP:0000286
      label: Epicanthus
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "broad nasal bridge, thin upper lip, hypertelorism, epicanthal folds,"
    explanation: >-
      Lists epicanthal folds among the commonly reported craniofacial features.
- category: Craniofacial
  name: Craniosynostosis
  description: >-
    Premature fusion of any cranial suture is part of the BLBS presentation and may
    require surgical management.
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "craniosynostosis (of any suture), and ocular involvement"
    explanation: >-
      GeneReviews includes craniosynostosis of any suture in the clinical
      description.
- category: Growth
  name: Microcephaly
  description: >-
    Head circumference at or below the 5th percentile in 30/95 (32%) of the
    combined cohort. Microcephaly is more common than macrocephaly across every
    stratification examined, and micro- or macrocephaly together affected 53% of
    H3-3B variant carriers.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microcephaly ( ≤5th percentile) 30/95 (32%)"
    explanation: >-
      Quantifies microcephaly at 30/95 (32%), within the FREQUENT band.
- category: Growth
  name: Macrocephaly
  description: >-
    A minority of individuals have head circumference at or above the 95th
    percentile (14/95, 15%); head size in BLBS may be small, large, or normal.
  phenotype_term:
    preferred_term: Macrocephaly
    term:
      id: HP:0000256
      label: Macrocephaly
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Macrocephaly ( ≥95th percentile) 14/95 (15%)"
    explanation: >-
      Quantifies macrocephaly at 14/95 (15%), within the OCCASIONAL band.
- category: Growth
  name: Short Stature
  description: >-
    Height at or below the 5th percentile in 32/91 (35%). Growth abnormality is
    somewhat less frequent with H3-3B (31%) than H3-3A variants.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Height ( ≤5th percentile) 32/91 (35%)"
    explanation: >-
      Quantifies short stature at 32/91 (35%), within the FREQUENT band.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"
    explanation: >-
      Independent cohort (including six H3-3B individuals) listing short stature
      among the major phenotypes.
- category: Gastrointestinal
  name: Failure to Thrive and Feeding Difficulties
  description: >-
    Poor growth and feeding problems are common; GeneReviews recommends feeding
    therapy and, in persistent cases, gastrostomy tube placement.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"
    explanation: >-
      Lists failure to thrive among the major phenotypes in a cohort that included
      six H3-3B individuals.
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Feeding issues and poor weight gain may benefit from feeding therapy"
    explanation: >-
      Confirms feeding issues and poor weight gain as recognised manifestations
      requiring intervention.
- category: Neurologic
  name: Structural Brain Abnormalities on Neuroimaging
  description: >-
    Abnormal brain MRI in 44/76 (58%) of individuals who underwent imaging.
    Findings include delayed myelination or hypomyelination, dysgenesis of the
    corpus callosum, dilated ventricles and hemispheric asymmetry.
  phenotype_term:
    preferred_term: Abnormal cerebral morphology
    term:
      id: HP:0002060
      label: Abnormal cerebral morphology
  frequency: FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fifty-eight percent of individuals with a reported MRI were diagnosed with at least one abnormal"
    explanation: >-
      Anchors the neuroimaging-findings row of the cohort phenotype table (44/76,
      58% of imaged individuals had at least one abnormal finding).
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "showed major phenotypes including global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment"
    explanation: >-
      Independent confirmation that structural brain abnormalities are a major
      phenotype in H3-3A/H3-3B variant carriers.
- category: Neurologic
  name: Small Posterior Fossa
  description: >-
    The most frequent single imaging finding in the dedicated neuroradiology
    series: 13 of 18 individuals (72%) with germline H3F3 variants had a small
    posterior fossa.
  phenotype_term:
    preferred_term: Small posterior fossa
    term:
      id: HP:0040010
      label: Small posterior fossa
  frequency: FREQUENT
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thirteen of 18 individuals (72%) presented with a small posterior fossa."
    explanation: >-
      Quantifies small posterior fossa at 13/18 (72%) in the imaging cohort.
- category: Neurologic
  name: Corpus Callosum Dysgenesis
  description: >-
    Malformation or dysgenesis of the corpus callosum in 28/76 (37%) of the
    combined cohort; in the dedicated imaging series 5 of 18 individuals (28%) had
    dysgenesis specifically of the splenium.
  phenotype_term:
    preferred_term: Aplasia/Hypoplasia of the corpus callosum
    term:
      id: HP:0007370
      label: Aplasia/Hypoplasia of the corpus callosum
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Corpus collosum malformation/dysgenesis 28/76 (37%)"
    explanation: >-
      Quantifies corpus callosum malformation at 28/76 (37%), within the FREQUENT
      band.
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Five individuals (28%) presented with dysgenesis of the splenium of the corpus callosum."
    explanation: >-
      Localises the callosal defect to the splenium in the imaging series.
- category: Neurologic
  name: Cortical Developmental Abnormalities
  description: >-
    Cortical malformations were noted in 8 of 18 individuals (44%) in the imaging
    series, most often dysgyria and hypoplastic temporal poles.
  phenotype_term:
    preferred_term: Abnormal cerebral cortex morphology
    term:
      id: HP:0002538
      label: Abnormal cerebral cortex morphology
  frequency: FREQUENT
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cortical developmental abnormalities were noted in 8 individuals (44%), with dysgyria and hypoplastic temporal poles being the most frequent presentation."
    explanation: >-
      Quantifies cortical developmental abnormalities at 8/18 (44%) and names the
      specific patterns.
- category: Neurologic
  name: Delayed Myelination
  description: >-
    Delayed myelination or hypomyelination is among the recurring MRI findings and
    may relate to the neural-crest-derived glial defect seen in the zebrafish
    model.
  phenotype_term:
    preferred_term: Delayed myelination
    term:
      id: HP:0012448
      label: Delayed myelination
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "included delayed myelination or hypomyelination; dysgenesis of the corpus collosum; dilated ventricles; and hemispheric asymmetry across multiple structures"
    explanation: >-
      Lists delayed myelination/hypomyelination among the common MRI findings.
- category: Neurologic
  name: Ventriculomegaly
  description: >-
    Dilated ventricles in 6/76 (8%); the imaging series additionally describes an
    enlarged and squared appearance of the frontal horns of the lateral ventricles
    with reduced, internally rotated caudate heads in a third of individuals.
  phenotype_term:
    preferred_term: Ventriculomegaly
    term:
      id: HP:0002119
      label: Ventriculomegaly
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Corpus collosum malformation/dysgenesis 28/76 (37%) Dilated ventricles 6/76 (8%) Asymmetry 4/76 (5%)"
    explanation: >-
      Quantifies ventricular dilatation at 6/76 (8%), within the OCCASIONAL band.
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Six individuals (33%) presented with reduced size and an internal rotational appearance of the heads of the caudate nuclei along with an enlarged and squared appearance of the frontal horns of the lateral ventricles."
    explanation: >-
      Describes the distinctive caudate/frontal-horn configuration reported in the
      imaging series.
- category: Ophthalmologic
  name: Oculomotor Dysfunction
  description: >-
    Abnormal eye movements affect 49/90 (54%) of individuals; most of those
    affected show eye rolling and strabismus, which may be secondary to abnormal
    muscle tone.
  phenotype_term:
    preferred_term: Abnormal eye physiology
    term:
      id: HP:0012373
      label: Abnormal eye physiology
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Oculomotor 49/90 (54%) Strabismus 32/90 (36%) Astigmatism 7/88 (8%)"
    explanation: >-
      Quantifies oculomotor dysfunction at 49/90 (54%), within the FREQUENT band.
- category: Ophthalmologic
  name: Strabismus
  description: Strabismus in 32/90 (36%) of individuals.
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Oculomotor 49/90 (54%) Strabismus 32/90 (36%)"
    explanation: >-
      Quantifies strabismus at 32/90 (36%), within the FREQUENT band.
- category: Ophthalmologic
  name: Astigmatism
  description: Astigmatism in 7/88 (8%) of individuals.
  phenotype_term:
    preferred_term: Astigmatism
    term:
      id: HP:0000483
      label: Astigmatism
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Strabismus 32/90 (36%) Astigmatism 7/88 (8%)"
    explanation: >-
      Quantifies astigmatism at 7/88 (8%), within the OCCASIONAL band.
- category: Musculoskeletal
  name: Scoliosis
  description: >-
    Scoliosis in 20/94 (21%), part of a broader musculoskeletal burden (60% of
    individuals) that also includes club foot, hip dysplasia, joint subluxation and
    femoral anteversion.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Musculoskeletal 56/94 (60%) Scoliosis 20/94 (21%)"
    explanation: >-
      Quantifies scoliosis at 20/94 (21%), within the OCCASIONAL band.
- category: Musculoskeletal
  name: Joint Hypermobility
  description: Ligamentous laxity in 21/94 (22%) of individuals.
  phenotype_term:
    preferred_term: Joint hypermobility
    term:
      id: HP:0001382
      label: Joint hypermobility
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lordosis/Kyphosis 4/94 (4%) Ligamentous laxity 21/94 (22%)"
    explanation: >-
      Quantifies ligamentous laxity at 21/94 (22%), within the OCCASIONAL band.
- category: Cardiovascular
  name: Congenital Heart Defects
  description: >-
    Cardiac anomalies, including atrial septal defects, in 11/82 (13%). They are
    about twice as likely with core-domain variants as with tail-domain variants.
  phenotype_term:
    preferred_term: Atrial septal defect
    term:
      id: HP:0001631
      label: Atrial septal defect
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cardiac 11/82 (13%) Dermal 46/88 (52%)"
    explanation: >-
      Quantifies cardiac anomalies at 11/82 (13%), within the OCCASIONAL band.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fourteen percent of individuals present with cardiac anomalies,"
    explanation: >-
      Confirms the cardiac-anomaly frequency in the narrative text.
- category: Genitourinary
  name: Genital Anomalies
  description: >-
    Genital anomalies in 20% of the combined cohort and specifically in 28% of
    H3-3B variant carriers versus 17% of H3-3A carriers, with cryptorchidism the
    typical finding in males. Some clinicians additionally reported renal anomalies
    (small, horseshoe or solitary kidney, nephrocalcinosis).
  phenotype_term:
    preferred_term: Abnormality of the genital system
    term:
      id: HP:0000078
      label: Abnormality of the genital system
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "duals with variants in H3-3B (28%) present with genital anomalies"
    explanation: >-
      Gene-specific frequency showing genital anomalies are enriched in H3-3B (BLBS2)
      carriers.
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Congenital anomalies are rare but can include congenital heart defects, brain malformations, and genitourinary abnormalities in males."
    explanation: >-
      GeneReviews confirms genitourinary abnormalities in males as part of the
      congenital-anomaly spectrum.
- category: Genitourinary
  name: Cryptorchidism
  description: >-
    Undescended testes are among the genital anomalies requiring intervention and
    are explicitly listed in the GeneReviews management recommendations.
  phenotype_term:
    preferred_term: Cryptorchidism
    term:
      id: HP:0000028
      label: Cryptorchidism
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "congenital heart defects, undescended testes, and hypothyroidism"
    explanation: >-
      GeneReviews lists undescended testes among the manifestations requiring
      standard treatment.
- category: Dermatologic
  name: Dermatologic Anomalies
  description: >-
    Just over half of individuals (46/88, 52%) have dermatologic findings such as
    eczema, hypoplastic nails, fetal finger pads, nipple abnormalities and 2/3 toe
    syndactyly.
  phenotype_term:
    preferred_term: Prominent fingertip pads
    term:
      id: HP:0001212
      label: Prominent fingertip pads
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fetal ﬁnger pad 14/88 (16%)"
    explanation: >-
      Quantifies fetal finger pads at 14/88 (16%), the specific dermatologic feature
      annotated here.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Dermal 46/88 (52%) Eczema 6/88 (7%) Nipple anomalies 15/88 (17%)"
    explanation: >-
      Gives the overall dermatologic burden described in this phenotype entry.
- category: Behavioral
  name: Autism Spectrum Disorder
  description: >-
    Co-existing autism spectrum disorder in 8% of individuals; attention deficit
    disorder and anxiety were each reported in 3%.
  phenotype_term:
    preferred_term: Autism
    term:
      id: HP:0000717
      label: Autism
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety"
    explanation: >-
      GeneReviews lists autism spectrum disorder among the manifestations requiring
      standard treatment.
- category: Endocrine
  name: Hypothyroidism
  description: >-
    Hypothyroidism is a recognised manifestation; GeneReviews recommends annual
    thyroid function testing as surveillance.
  phenotype_term:
    preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "congenital heart defects, undescended testes, and hypothyroidism"
    explanation: >-
      GeneReviews lists hypothyroidism among the manifestations requiring standard
      treatment, and recommends annual thyroid function tests.
- category: Gastrointestinal
  name: Constipation
  description: >-
    Constipation is common enough to warrant monitoring at every clinical visit per
    the GeneReviews surveillance schedule.
  phenotype_term:
    preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "monitor for constipation; assess for new manifestations, such as seizures and changes in tone and/or gait"
    explanation: >-
      GeneReviews includes constipation monitoring in the routine surveillance
      schedule, implying it is a recurrent problem.
- category: Neurologic
  name: Hearing Impairment
  description: >-
    Hearing loss is a recognised manifestation warranting annual audiology
    evaluation.
  phenotype_term:
    preferred_term: Hearing impairment
    term:
      id: HP:0000365
      label: Hearing impairment
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "craniosynostosis, vision issues, hearing loss, congenital heart defects"
    explanation: >-
      GeneReviews lists hearing loss among the manifestations requiring standard
      treatment.
- category: Neurologic
  name: Delayed Ability to Walk
  description: >-
    Delayed or unachieved independent walking (beyond 20 months) in 59/75 (79%) of
    the combined cohort; less frequent among H3-3B carriers (65%) than H3-3A
    carriers (85%).
  phenotype_term:
    preferred_term: Delayed ability to walk
    term:
      id: HP:0031936
      label: Delayed ability to walk
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Delayed/No walking (>20 months) 59/75 (79%)"
    explanation: >-
      Quantifies delayed or unachieved walking at 59/75 (79%), within the FREQUENT
      band.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "independent walking when compared to individuals harboring variants in H3-3B (65%)"
    explanation: >-
      Gives the H3-3B-specific (BLBS2) figure for delayed or unachieved independent
      walking.
- category: Neurologic
  name: Delayed Speech and Language Development
  description: >-
    Only 60% (50/84) of individuals had spoken at least one word beyond 20 months of
    age, so most have significant expressive language delay.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Speaks at least one word (>20 months) 50/84 (60%)"
    explanation: >-
      The complement of this figure (40% not speaking at all beyond 20 months, with
      further delay among those who do speak) supports frequent speech delay.
genetic:
- name: H3-3B
  gene_term:
    preferred_term: H3-3B
    term:
      id: hgnc:4765
      label: H3-3B
  association: De novo heterozygous pathogenic variants
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    H3-3B (formerly H3F3B) is the sole cause of BLBS2. It encodes the
    replication-independent histone variant H3.3, a protein identical to that
    encoded by H3-3A; the two genes differ in untranslated and coding sequence,
    giving different expression patterns and levels, and both are expressed
    ubiquitously with relatively high levels in brain, testes, ovaries and uterus.
    Both genes are strongly missense-constrained (gnomAD missense Z = 2.95 for
    H3F3B), and cohort variants were essentially absent from 138,632 gnomAD
    controls. Reported BLBS2 alleles include missense changes in both the histone
    tail and the histone core (for example p.A29P, p.G34V, p.L48R, p.M121I,
    p.P121R, p.R129H), a synonymous variant that alters an alternate-transcript
    stop codon (p.V117V/p.S146X), and stop-loss variants; p.P121R was recurrent in
    the founding cohort. Variant location within the protein — and the identity of
    the gene (H3-3A versus H3-3B) — contributes more to phenotypic severity than
    sex, but no definitive genotype-phenotype correlation has been established, and
    individuals with the same variant can differ markedly.
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bryant-Li-Bhoj syndrome (BLBS), which became OMIM-classified in 2022 (OMIM: 619720, 619721), is caused by germline variants in the two genes that encode histone H3.3 (H3-3A/H3F3A and H3-3B/H3F3B)"
    explanation: >-
      Establishes H3-3B as one of the two BLBS genes and ties it to the OMIM
      entities, of which 619721 is BLBS2.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we report eleven de novo missense variants and one de novo stop-loss variant in H3-3A (n = 6) and H3-3B (n = 6)"
    explanation: >-
      Documents six independent de novo H3-3B variants, giving direct BLBS2-specific
      genetic evidence.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the gnomAD missense Z score is 3.21 for H3F3A and 2.95 for H3F3B (>2 is significant)"
    explanation: >-
      Establishes strong missense constraint on H3F3B, supporting pathogenicity of de
      novo missense alleles in this gene.
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the results presented here suggest that the location of the variants within the H3.3 protein and the affected gene (H3-3A or H3-3B) contribute more to the severity of distinct phenotypes than sex"
    explanation: >-
      Establishes gene identity and variant position, not sex, as the main identified
      modifiers of severity.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Both H3F3B and H3F3A are expressed ubiquitously, during development and throughout life, with relatively high levels of expression in the brain, testes, ovaries, and uterus"
    explanation: >-
      Expression pattern of H3-3B matching the neurologic and genitourinary organ
      involvement of BLBS2.
variants:
- name: H3-3B p.Leu48Arg
  description: >-
    The BLBS2 allele modelled in the isogenic iPSC system. It increases H3-3B
    expression and drives hyper-deposition of H3.3 into nucleosomes, with
    downstream changes in chromatin accessibility, neuronal fate, adhesion,
    neurotransmission and excitatory/inhibitory balance. Bryant et al. classify
    p.L48R structurally as a variant that may alter the interaction of H3.3 with
    other nucleosomal histones.
  gene:
    preferred_term: H3-3B
    term:
      id: hgnc:4765
      label: H3-3B
  evidence:
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we investigate how one BLBS-causative variant, H3-3B p.Leu48Arg (L48R), affects neurodevelopment"
    explanation: >-
      Identifies p.Leu48Arg as a BLBS-causative H3-3B allele and the subject of the
      mechanistic model.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "they may alter the interaction of H3.3 with other nucleosomal histones (e.g., p.L48R)"
    explanation: >-
      Structural interpretation of p.L48R as a variant disrupting histone-histone
      contacts within the nucleosome.
- name: H3-3B p.Gly34Val
  description: >-
    A germline BLBS2 allele at a residue that is also mutated somatically in
    pediatric high-grade glioma. In the somatic setting p.G34V drives oncogenesis
    only in combination with ATRX/DAXX and TP53 lesions; in the germline setting
    carriers develop BLBS without malignancy, and patient fibroblasts show
    increased proliferation. This allele is the clearest illustration of the
    germline-versus-somatic mechanistic divergence in H3.3 disease.
  gene:
    preferred_term: H3-3B
    term:
      id: hgnc:4765
      label: H3-3B
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The only other variant in our cohort that is similar to a cancer-linked variant is the p.G34V mutation in H3F3B."
    explanation: >-
      Identifies the germline H3F3B p.G34V allele and its relationship to the
      cancer-associated substitution at the same residue.
- name: H3-3B p.Pro121Arg
  description: >-
    A recurrent BLBS2 allele, found in two unrelated individuals with H3-3B
    variants in the founding cohort (and in one further individual as the
    corresponding H3-3A change). Patient lymphoblasts carrying this allele were
    among those used to demonstrate the local histone PTM disturbance.
  gene:
    preferred_term: H3-3B
    term:
      id: hgnc:4765
      label: H3-3B
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "p.Q125R was found in H3F3A in three individuals and in a different individual in H3F3B, while p.P121R was found in H3F3A in one individual and in H3F3B in two other individuals."
    explanation: >-
      Documents p.P121R as recurrent in H3F3B, one of the few non-private BLBS2
      alleles.
diagnosis:
- name: Molecular genetic testing
  description: >-
    The diagnosis of BLBS2 is established in a proband with suggestive clinical
    findings and a heterozygous pathogenic variant in H3-3B identified by molecular
    genetic testing. Because the clinical presentation is nonspecific and the
    causative alleles are largely private, exome or genome sequencing (or a broad
    neurodevelopmental panel including H3-3A and H3-3B) is the practical diagnostic
    route; the two BLBS entities cannot be distinguished clinically and are
    separated only by which gene carries the variant.
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of BRYLIB is established in a proband with suggestive findings and a heterozygous pathogenic variant in either H3-3A (H3F3A) or H3-3B (H3F3B) identified by molecular genetic testing."
    explanation: >-
      States the diagnostic criterion; for BLBS2 the qualifying gene is H3-3B.
- name: Brain magnetic resonance imaging
  description: >-
    Brain MRI supports the diagnosis and characterises the structural phenotype.
    The recognisable pattern comprises a small posterior fossa, dysgenesis of the
    corpus callosum (especially the splenium), cortical developmental abnormalities
    (dysgyria, hypoplastic temporal poles), deformity of the lateral ventricles
    with reduced internally rotated caudate heads, and delayed myelination. MRI is
    abnormal in roughly 58% of imaged individuals, so a normal scan does not
    exclude the diagnosis.
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumor Development"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Imaging phenotypes in germline H3F3-affected individuals are related to brain features, including a small posterior fossa as well as dysgenesis of the corpus callosum, cortical developmental abnormalities, and deformity of lateral ventricles."
    explanation: >-
      Defines the imaging pattern that supports a clinical suspicion of BLBS.
treatments:
- name: Developmental and Rehabilitative Therapy
  description: >-
    There is no disease-modifying therapy for BLBS2. Management is supportive and
    follows standard practice for developmental delay/intellectual disability, with
    early intervention, physical, occupational and speech therapy directed at motor
    and communication milestones.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Rehabilitation
    term:
      id: NCIT:C15315
      label: Rehabilitation
  target_phenotypes:
  - preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for developmental delay / intellectual disability"
    explanation: >-
      GeneReviews recommends standard developmental management; no targeted therapy
      exists.
- name: Feeding Therapy and Gastrostomy
  description: >-
    Feeding issues and poor weight gain are managed with feeding therapy; a
    gastrostomy tube may be considered when feeding problems persist.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Feeding issues and poor weight gain may benefit from feeding therapy; in individuals with persistent feeding issues, gastrostomy tube may be considered."
    explanation: >-
      Directly states the feeding-management recommendation.
- name: Antiseizure Pharmacotherapy
  description: >-
    Epilepsy affects a majority of individuals with H3-3B variants and is managed
    with standard antiseizure medication; no BLBS-specific regimen or
    contraindication has been established.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_phenotypes:
  - preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation"
    explanation: >-
      GeneReviews directs standard treatment for epilepsy among the manifestations of
      BLBS. The snippet begins at the recommendation clause so it reads as a
      management statement rather than a list of features.
- name: Craniosynostosis Surgery
  description: >-
    Craniosynostosis of any suture may occur and is managed with standard surgical
    correction.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: Surgical Procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_phenotypes:
  - preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation, craniosynostosis, vision issues"
    explanation: >-
      GeneReviews lists craniosynostosis among the manifestations receiving standard
      treatment, which is surgical. The snippet begins at the recommendation clause so
      it reads as a management statement rather than a list of features.
- name: Multisystem Surveillance
  description: >-
    Recommended surveillance includes, at each visit, growth parameters, nutrition
    and oral intake safety, constipation, new manifestations such as seizures and
    changes in tone or gait, developmental progress and behaviour; and annually or
    as indicated, ophthalmology evaluation, audiology evaluation and thyroid
    function tests. Routine tumour surveillance is NOT recommended: the
    community-wide cancer incidence across germline histonopathies is currently 1%,
    below the 5% threshold used to justify a surveillance protocol.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Annually or as clinically indicated: ophthalmology evaluation, audiology evaluation, and thyroid function tests."
    explanation: >-
      Specifies the annual surveillance schedule.
  - reference: PMID:40241305
    reference_title: "Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"
    explanation: >-
      Evidence for the explicit recommendation against routine tumour surveillance.
- name: Genetic Counseling
  description: >-
    Counselling covers the autosomal dominant, typically de novo origin, the
    consequently low sibling recurrence risk, and the availability of prenatal and
    preimplantation genetic testing once the familial H3-3B variant is known.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome (GeneReviews)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Once an H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible."
    explanation: >-
      States the reproductive options that genetic counselling covers.
differential_diagnoses:
- name: Bryant-Li-Bhoj neurodevelopmental syndrome 1
  description: >-
    The H3-3A-related counterpart of this disorder. The two are clinically
    indistinguishable at the level of an individual patient — the protein product
    is identical — and are separated only by which paralog carries the variant.
    Cohort-level differences favour BLBS1 for delayed walking (85% vs 65%) and
    dysmorphism, and BLBS2 for seizures (59% vs 45%) and genital anomalies (28% vs
    17%), but these are group trends, not diagnostic discriminators.
  disease_term:
    preferred_term: Bryant-Li-Bhoj neurodevelopmental syndrome 1
    term:
      id: MONDO:0030606
      label: Bryant-Li-Bhoj neurodevelopmental syndrome 1
  distinguishing_features:
  - "Determined solely by molecular testing: a pathogenic variant in H3-3A rather than H3-3B"
  - Cohort-level trend toward more frequent delayed walking and dysmorphism than in BLBS2
  - Cohort-level trend toward fewer seizures and fewer genital anomalies than in BLBS2
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While specific genotype-phenotype correlations have not been conclusively delineated"
    explanation: >-
      Confirms that no established genotype-phenotype rule separates the two entities
      clinically.
- name: Diffuse midline glioma, H3 K27-altered
  description: >-
    The somatic counterpart of H3.3 disease. Recurrent somatic H3.3 substitutions
    (K27M, G34R/V) in glial precursors drive pediatric high-grade glioma with
    genome-wide collapse of histone post-translational modifications. Germline BLBS
    variants are private, scattered, cause only local PTM changes, and are not
    associated with malignancy — the 46-patient founding cohort had congenital
    anomalies without tumours.
  distinguishing_features:
  - Somatic and tumour-restricted rather than germline and constitutional
  - Recurrent hotspot substitutions (K27M, G34R/V) rather than private protein-wide variants
  - Global histone PTM dysregulation rather than local in-cis PTM change
  - Malignancy as the presenting problem rather than a congenital neurodevelopmental syndrome
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
    explanation: >-
      Contrasts the germline (local) and somatic (global) epigenetic mechanisms that
      separate BLBS from the oncohistone diseases.
experimental_models:
- name: H3-3B p.Leu48Arg iPSC-derived neural model
  description: >-
    Induced pluripotent stem cells carrying the BLBS2 allele H3-3B p.Leu48Arg,
    differentiated into 2D neural progenitor cells, 2D forebrain neurons and 3D
    dorsal forebrain organoids, with paired transcriptomic and
    chromatin-accessibility profiling, immunofluorescence cell-identity mapping and
    whole-cell patch clamp. This is the first human-derived mechanistic model of
    BLBS and the principal BLBS2-specific model system available.
  experimental_model_type: IPSC_DERIVED_MODEL
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  publication: PMID:41689096
  findings:
  - statement: >-
      Paired chromatin-accessibility and transcriptomic changes affect neuronal
      fate, cell adhesion, neurotransmission and excitatory/inhibitory balance.
    supporting_text: "In the 2D systems, we found dysregulated gene expression and chromatin accessibility affecting neuronal fate, adhesion, neurotransmission, and excitatory/inhibitory balance."
  - statement: >-
      Organoids show altered proportions of radial glia and mature neurons, and the
      neurons that form are electrophysiologically immature.
    supporting_text: "Patch clamp recordings revealed decreased electrical activity in neurons from L48R DFBOs compared to control DFBOs."
  - statement: >-
      The proposed proximal mechanism is increased H3-3B expression driving
      hyper-deposition of H3.3 into the nucleosome.
    supporting_text: "H3.3 L48R increases H3-3B expression, resulting in the hyper-deposition of H3.3 into the nucleosome, which underlies changes in gene expression and chromatin accessibility"
  evidence:
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"
    explanation: >-
      Establishes the model's standing as the first human-derived mechanistic system
      for BLBS.
animal_models:
- species: Danio rerio
  genotype: h3f3a p.D123N dominant allele; h3f3a db1092 homozygous null
  description: >-
    A dominant zebrafish h3f3a p.D123N allele from a forward genetic screen sits two
    residues from a patient variant (p.Q125R) and reproduces the dominant
    inheritance and craniofacial anomalies of BLBS. Homozygous h3f3a db1092 mutants
    completely lose neural-crest-derived jaw cartilage and show reduced
    foxd3-positive neural-crest-derived glia, melanocytes and xanthophores. The
    model uses the h3f3a paralog rather than an h3f3b orthologue, so it is
    informative for the shared H3.3 mechanism rather than for BLBS2 specifically.
  associated_phenotypes:
  - Craniofacial anomalies
  - Loss of neural-crest-derived jaw cartilage
  - Reduced foxd3-positive neural-crest-derived glia
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia."
    explanation: >-
      Summarises the zebrafish phenotype recapitulating the craniofacial and glial
      components of BLBS.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "This heterozygous missense variant replicates the dominant inheritance observed in humans and is only two amino acids away from a mutation identified in an affected patient (p.Q125R)."
    explanation: >-
      Anchors the statement that the dominant zebrafish allele replicates the
      dominant inheritance observed in human BLBS.
discussions:
- discussion_id: blbs2_paralog_basis
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Why do variants in H3-3B produce a nosologically separate disorder (BLBS2) from
    variants in H3-3A (BLBS1) when the two genes encode an identical protein?
  attaches_to:
  - pathophysiology#De Novo H3-3B Variant Altering Histone H3.3
  rationale: >-
    The two BLBS entities exist because two genes are involved, not because two
    proteins are. The only known differences are in untranslated and coding
    sequence, which alter expression pattern and level. Cohort data show
    gene-dependent frequency differences (seizures 59% vs 45%; genital anomalies 28%
    vs 17%; delayed walking 65% vs 85%), so the paralogs are not interchangeable,
    but the mechanism converting a paralog-level expression difference into these
    phenotype shifts is unknown. Resolving it determines whether BLBS1 and BLBS2
    should remain separate entities or be lumped.
  proposed_experiments:
  - experiment_id: exp_blbs2_paralog_expression_atlas
    name: Paralog-resolved H3-3A/H3-3B expression atlas across human brain development
    description: >-
      Quantify H3-3A and H3-3B transcript and protein abundance across human
      developmental brain stages and cell types to test whether the paralogs differ
      in the windows and lineages relevant to the divergent phenotypes.
    would_support:
    - "Paralog-specific expression differences in neural lineages would explain why an identical protein product yields two clinical entities."
    would_refute:
    - "Indistinguishable paralog expression across all relevant stages and cell types would argue that the BLBS1/BLBS2 split is nosological rather than biological."
  - experiment_id: exp_blbs2_isogenic_paralog_swap
    name: Isogenic paralog-matched substitution series in iPSC-derived neurons
    description: >-
      Generate isogenic iPSC lines carrying the same amino-acid substitution
      introduced into H3-3A versus H3-3B, then compare chromatin accessibility,
      transcriptome and neuronal electrophysiology between the matched pairs.
    would_support:
    - "Divergent chromatin or electrophysiological phenotypes from the same substitution in the two paralogs would establish a paralog-intrinsic effect."
    would_refute:
    - "Identical phenotypes from matched substitutions would locate the difference in mutational opportunity or ascertainment rather than paralog biology."
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since these variables do not account for all BLBS phenotypic variability, these findings suggest that additional factors may play a role in modifying the phenotypes of affected individuals."
    explanation: >-
      The authors explicitly state that gene, variant location and sex leave BLBS
      phenotypic variability unexplained.
- discussion_id: blbs2_variant_heterogeneity
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Do all BLBS2 alleles act through a single mechanism (H3.3 hyper-deposition), or
    do tail, core and stop-loss variants act through distinct local mechanisms?
  attaches_to:
  - pathophysiology#Disrupted Nucleosome Assembly and Chaperone Interaction
  rationale: >-
    Functional data point in different directions: some constructs reduce H3.3
    protein levels while another accumulates above wild-type; one increases
    chaperone binding; and the one allele studied in a human neural model (p.L48R)
    increases H3-3B expression and nucleosomal H3.3. Whether these are variations on
    one theme or genuinely different mechanisms determines whether a single
    therapeutic strategy could serve the whole disorder.
  proposed_experiments:
  - experiment_id: exp_blbs2_allelic_series_multiomic
    name: Multi-omic iPSC allelic series across tail, core and stop-loss BLBS2 variants
    description: >-
      Apply the p.L48R iPSC pipeline to a panel of BLBS2 alleles spanning the histone
      tail, the histone core and stop-loss classes, quantifying nucleosomal H3.3
      occupancy genome-wide alongside transcriptome and chromatin accessibility.
    would_support:
    - "Shared hyper-deposition across allele classes would establish a single unifying mechanism and a single therapeutic target."
    would_refute:
    - "Opposite-direction deposition changes across allele classes would confirm mechanistic heterogeneity and argue against a one-size-fits-all therapy."
  evidence:
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."
    explanation: >-
      Demonstrates opposite-direction effects on H3.3 abundance among BLBS alleles,
      which is the substance of this gap.
- discussion_id: blbs2_proliferation_no_cancer
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Why do germline H3-3B variant carriers show increased cellular proliferation yet
    no excess of malignancy, including at residues (p.G34) that are oncogenic when
    mutated somatically?
  attaches_to:
  - pathophysiology#Deregulated Cell Proliferation
  rationale: >-
    Patient fibroblasts, including H3F3B p.G34V lines, proliferate faster than
    controls and up-regulate mitosis genes, yet the founding 46-patient cohort had
    congenital anomalies without malignancies and the community-wide cancer
    incidence across germline histonopathies sits at 1%. Somatic p.G34V is oncogenic
    only in combination with ATRX/DAXX and TP53 lesions, suggesting the germline
    context lacks the required cooperating hits, or that developmental timing and
    cell of origin gate transformation.
  proposed_experiments:
  - experiment_id: exp_blbs2_cancer_incidence_followup
    name: Longitudinal cancer-incidence follow-up of the international BLBS cohort
    description: >-
      Prospectively track tumour incidence in the registry cohort with standardised
      ascertainment, extending the community-wide incidence estimate and testing it
      against the 5% threshold used to justify routine surveillance.
    would_support:
    - "An incidence rising above the surveillance threshold would establish BLBS2 as a cancer-predisposition syndrome."
    would_refute:
    - "A stable incidence at or below population background would confirm that no tumour surveillance is warranted."
  - experiment_id: exp_blbs2_cooperating_hit_transformation
    name: Cooperating-lesion transformation assay in germline H3.3-variant cells
    description: >-
      Test whether germline H3.3-variant cells acquire transformation potential when
      ATRX/DAXX or TP53 are additionally disrupted, and compare the chromatin state
      of germline-variant glial precursors with K27M/G34-mutant glioma cells.
    would_support:
    - "Transformation only in the presence of cooperating lesions would explain the germline proliferation-without-malignancy paradox."
    would_refute:
    - "Failure to transform under any cooperating lesion would point instead to cell of origin or developmental timing as the gate."
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."
    explanation: >-
      States the unresolved convergence between germline and somatic H3.3 mechanisms
      that motivates this gap.
  - reference: PMID:40241305
    reference_title: "Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the value of ongoing translational work to address the outstanding question of cancer predisposition for individuals living with germline histonopathies"
    explanation: >-
      The pan-histonopathy cohort names cancer predisposition as an explicitly
      outstanding question.
- discussion_id: blbs2_neurodegeneration_extent
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Is BLBS2 genuinely neurodegenerative, or is the apparent regression a
    developmental plateau with musculoskeletal decline?
  attaches_to:
  - pathophysiology#Abnormal Brain Development and Progressive Neurologic Dysfunction
  rationale: >-
    BLBS was first described as a progressive neurodegenerative syndrome, and
    GeneReviews lists loss of milestones, worsening gait, spasticity and
    camptocormia. But the expanded cohort reframed the disorder as primarily
    neurodevelopmental, and some individuals show resolving rather than progressive
    hypotonia. No longitudinal imaging or biomarker study has established whether
    neuronal loss actually occurs. The answer changes both prognostic counselling
    and trial endpoint selection.
  proposed_experiments:
  - experiment_id: exp_blbs2_longitudinal_imaging_biomarker
    name: Prospective longitudinal MRI volumetry with neuroaxonal injury biomarkers
    description: >-
      Follow a BLBS2 cohort with serial MRI volumetry, standardised developmental
      assessment and serial neurofilament light chain measurement to distinguish
      genuine neuronal loss from a developmental plateau.
    would_support:
    - "Progressive volume loss with rising neurofilament light chain would confirm a true neurodegenerative component."
    would_refute:
    - "Stable volumes and biomarkers alongside a developmental plateau would reframe the apparent regression as non-degenerative."
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "BLBS was initially categorized as a progressive neurodegenerative syndrome caused by de novo heterozygous variants in either H3-3A or H3-3B"
    explanation: >-
      Documents the initial neurodegenerative framing that the expanded
      neurodevelopmental characterisation now sits alongside.
  - reference: PMID:40241305
    reference_title: "Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "histonopathy-associated phenotypes change throughout the lifespan, necessitating longitudinal re-evaluation"
    explanation: >-
      Independent argument that the natural history is not yet settled and requires
      longitudinal study.
- discussion_id: blbs2_human_model_fidelity
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Does the available zebrafish evidence for BLBS reflect BLBS2 biology, given that
    it models the h3f3a paralog rather than h3f3b?
  attaches_to:
  - pathophysiology#Neural Crest and Craniofacial Developmental Defect
  rationale: >-
    The craniofacial and glial arms of the mechanism rest substantially on a
    zebrafish h3f3a model (dominant p.D123N and homozygous db1092 alleles). Since
    the whole basis for splitting BLBS1 from BLBS2 is a paralog-level difference in
    expression rather than protein sequence, a model built on the other paralog
    cannot by construction test paralog-specific effects, and zebrafish h3f3
    gene-family structure differs from the human two-gene arrangement. The
    craniofacial phenotype may therefore be attributable to total H3.3 dosage rather
    than to anything specific to H3-3B.
  proposed_experiments:
  - experiment_id: exp_blbs2_h3f3b_orthologue_models
    name: Orthologous h3f3b animal models for the craniofacial and glial arms
    description: >-
      Generate zebrafish and mouse models of orthologous h3f3b alleles, compare
      craniofacial and glial phenotypes with the existing h3f3a models, and quantify
      paralog-specific expression in neural crest across species.
    would_support:
    - "Reproduction of the craniofacial and foxd3-glia phenotypes by h3f3b alleles would license the existing model evidence for BLBS2."
    would_refute:
    - "Absence of the phenotype in h3f3b models would confine the current model evidence to total H3.3 dosage rather than to BLBS2."
  - experiment_id: exp_blbs2_ipsc_neural_crest
    name: Human iPSC-derived neural crest test of the foxd3-glia phenotype
    description: >-
      Differentiate BLBS2-allele iPSCs to neural crest and crest-derived glia to test
      whether the zebrafish foxd3-positive glial deficit reproduces in human cells.
    would_support:
    - "A crest-derived glial deficit in human BLBS2 cells would bridge the cross-species gap."
    would_refute:
    - "Normal human crest-derived glia would leave the zebrafish glial phenotype unconfirmed for human disease."
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A dominant zebrafish model (p.D123N) derived from a forward genetic screen has been previously reported with craniofacial abnormalities, which were replicated in our patient cohort"
    explanation: >-
      Confirms that the craniofacial model evidence comes from the h3f3a paralog,
      which is the substance of the translational mismatch.
references:
- reference: PMID:37782742
  title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome"
  tags:
  - GeneReviews
  findings:
  - statement: >-
      Expert-curated clinical description, diagnostic criteria, management and
      surveillance recommendations, and genetic counselling guidance for BLBS
      (covering both the H3-3A and H3-3B entities).
    supporting_text: "The diagnosis of BRYLIB is established in a proband with suggestive findings and a heterozygous pathogenic variant in either H3-3A (H3F3A) or H3-3B (H3F3B) identified by molecular genetic testing."
- reference: PMID:33268356
  title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients"
  findings:
  - statement: >-
      Founding 46-patient cohort defining the disorder, with molecular modelling of
      37 variants, patient histone PTM profiling, patient-cell RNA-seq and
      proliferation assays, and a zebrafish craniofacial/neural-crest-glia model.
    supporting_text: "We analyze 46 patients bearing de novo germline mutations in histone 3 family 3A (H3F3A) or H3F3B with progressive neurologic dysfunction and congenital anomalies without malignancies."
- reference: PMID:38678163
  title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals"
  findings:
  - statement: >-
      Combined 96-person cohort supplying the quantitative phenotype table used
      throughout this entry, plus the gene-stratified (H3-3A versus H3-3B) and
      variant-location analyses.
    supporting_text: "In this larger cohort of 96 people, we identify causative missense, synonymous, and stop-loss variants."
- reference: PMID:34876591
  title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities"
  findings:
  - statement: >-
      Independent cohort of twelve de novo variants (six in H3-3B) with functional
      characterisation of protein expression and DAXX chaperone binding.
    supporting_text: "Here we report eleven de novo missense variants and one de novo stop-loss variant in H3-3A (n = 6) and H3-3B (n = 6)"
- reference: PMID:41689096
  title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation"
  findings:
  - statement: >-
      First human-derived mechanistic model, built on the H3-3B p.L48R allele, with
      multi-omic and electrophysiological characterisation supporting the
      hyper-deposition model.
    supporting_text: "These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"
- reference: PMID:35772801
  title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
  findings:
  - statement: >-
      Dedicated neuroradiology series of 18 individuals defining the BLBS imaging
      signature.
    supporting_text: "Eighteen individuals (10 males, 56%) with H3F3 germline variants were included."
- reference: PMID:36867246
  title: "Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species"
  findings:
  - statement: >-
      Review collating cross-species functional data on H3.3 missense variants and
      articulating the gain-of-function/dominant-negative model.
    supporting_text: "Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."
- reference: PMID:40241305
  title: "Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies"
  findings:
  - statement: >-
      Pan-histonopathy cohort of 192 individuals; establishes the 1% community-wide
      cancer incidence that argues against routine tumour surveillance, and finds
      weaker support for several non-core BLBS features.
    supporting_text: "the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"
- reference: PMID:40696776
  title: "Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene"
  findings:
  - statement: >-
      Single-case report extending the BLBS2 movement-disorder phenotype.
    supporting_text: "Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene."
notes: >-
  Entity scope: this entry covers BLBS2 (H3-3B / OMIM 619721) only. Much of the
  published evidence is reported for the combined BLBS cohort across both genes,
  because the two paralogs encode an identical protein and were ascertained
  together. Where the sources report gene-stratified figures, the H3-3B-specific
  number is quoted; where they do not, the combined-cohort figure is used and
  labelled as such. The H3-3A-specific counterpart (BLBS1, MONDO:0030606) is
  recorded as a differential diagnosis rather than folded in here. No prevalence
  estimate is recorded because none has been published for BLBS overall or for
  BLBS2 specifically; roughly 100 individuals have been reported across both
  genes.

📚

References & Deep Research

References

9

PMID:37782742

Bryant-Li-Bhoj Neurodevelopmental Syndrome

1 finding

Expert-curated clinical description, diagnostic criteria, management and surveillance recommendations, and genetic counselling guidance for BLBS (covering both the H3-3A and H3-3B entities).

"The diagnosis of BRYLIB is established in a proband with suggestive findings and a heterozygous pathogenic variant in either H3-3A (H3F3A) or H3-3B (H3F3B) identified by molecular genetic testing."

PMID:33268356

Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients

1 finding

Founding 46-patient cohort defining the disorder, with molecular modelling of 37 variants, patient histone PTM profiling, patient-cell RNA-seq and proliferation assays, and a zebrafish craniofacial/neural-crest-glia model.

"We analyze 46 patients bearing de novo germline mutations in histone 3 family 3A (H3F3A) or H3F3B with progressive neurologic dysfunction and congenital anomalies without malignancies."

PMID:38678163

Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals

1 finding

Combined 96-person cohort supplying the quantitative phenotype table used throughout this entry, plus the gene-stratified (H3-3A versus H3-3B) and variant-location analyses.

"In this larger cohort of 96 people, we identify causative missense, synonymous, and stop-loss variants."

PMID:34876591

De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities

1 finding

Independent cohort of twelve de novo variants (six in H3-3B) with functional characterisation of protein expression and DAXX chaperone binding.

"Here we report eleven de novo missense variants and one de novo stop-loss variant in H3-3A (n = 6) and H3-3B (n = 6)"

PMID:41689096

A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation

1 finding

First human-derived mechanistic model, built on the H3-3B p.L48R allele, with multi-omic and electrophysiological characterisation supporting the hyper-deposition model.

"These data provide the first mechanistic insights into the pathogenesis of BLBS from a human-derived model of neurodevelopment"

PMID:35772801

Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors.

1 finding

Dedicated neuroradiology series of 18 individuals defining the BLBS imaging signature.

"Eighteen individuals (10 males, 56%) with H3F3 germline variants were included."

PMID:36867246

Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species

1 finding

Review collating cross-species functional data on H3.3 missense variants and articulating the gain-of-function/dominant-negative model.

"Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."

PMID:40241305

Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies

1 finding

Pan-histonopathy cohort of 192 individuals; establishes the 1% community-wide cancer incidence that argues against routine tumour surveillance, and finds weaker support for several non-core BLBS features.

"the community-wide cancer incidence is currently 1%, which falls below the recommended 5% cut off for routine surveillance"

PMID:40696776

Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene

1 finding

Single-case report extending the BLBS2 movement-disorder phenotype.

"Paroxysmal Dyskinesia with a Novel Variant in the Histone 3 Family 3B (H3-3B) Gene."

Deep Research

1

Falcon ▸

Bryant-Li-Bhoj Neurodevelopmental Syndrome 2: Disease-Characteristics Report

Edison Scientific Literature 12 citations 2026-08-01T17:21:37.790227

Bryant-Li-Bhoj Neurodevelopmental Syndrome 2: Disease-Characteristics Report

Executive summary

Bryant-Li-Bhoj neurodevelopmental syndrome 2 (BLBNS2) is an ultra-rare Mendelian chromatin disorder caused by heterozygous germline variants in H3F3B (current symbol H3-3B), one of two genes encoding the replication-independent histone H3.3 protein. It is the H3-3B-associated subtype of Bryant-Li-Bhoj syndrome (BLBS); the corresponding H3-3A-associated disorder is type 1. The best-supported phenotype comprises developmental delay/intellectual disability, motor and speech delay, tone abnormalities, dysmorphic craniofacial features, abnormal growth, visual impairment, and variably abnormal brain MRI. Some patients develop regression, progressive ataxia, white-matter disease, or cerebral atrophy, but progression is neither universal nor adequately quantified for type 2 alone. Most cases arise from de novo dominant variants, although inherited H3-3B disease has now been documented. No disease-modifying therapy, validated biomarker, formal diagnostic criteria, prevalence estimate, or BLBNS2-specific clinical trial is available.

A central evidence limitation is that most clinical publications pool H3-3A and H3-3B cases. This report therefore labels findings as H3-3B-specific or combined BLBS and does not treat pooled percentages as type-2-specific rates.

domain	best-supported finding	evidence scope (H3F3B-specific versus combined BLBS)	ontology/identifier suggestion	evidence limitation
Disease identity	Bryant-Li-Bhoj neurodevelopmental syndrome 2 is the H3F3B-associated subtype of Bryant-Li-Bhoj syndrome; OMIM-classified in 2022 and mapped in Open Targets to MONDO:0030607 (layocarris2024expandedphenotypicspectrum pages 3-4, OpenTargets Search: Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B)	H3F3B-specific disease label supported by database mapping; phenotype literature often combines H3F3A and H3F3B	MONDO:0030607; OMIM: 619721; gene: H3F3B / H3-3B	Many publications discuss combined BLBS rather than isolated type 2
Synonyms/nomenclature	Names in current use include Bryant-Li-Bhoj syndrome (BLBS) for the combined disorder and Bryant-Li-Bhoj neurodevelopmental syndrome 2 for the H3F3B form (layocarris2024expandedphenotypicspectrum pages 3-4, OpenTargets Search: Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B)	Mixed	Bryant-Li-Bhoj neurodevelopmental syndrome 2; BLBS	No stable broad synonym set beyond OMIM/database naming located in the retrieved evidence
Causal gene	Causal gene is H3F3B (also written H3-3B), encoding histone H3.3 histone B (layocarris2024expandedphenotypicspectrum pages 3-4, OpenTargets Search: Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B, okur2021denovovariants pages 2-3)	H3F3B-specific	H3F3B; H3-3B	Some papers use H3-3B and H3F3B interchangeably
Inheritance	Most reported cases are heterozygous germline de novo variants; inheritance pattern is best described as autosomal dominant, with newer evidence showing that not all cases are strictly de novo because maternally inherited p.N108S has been reported in the expanded cohort (layocarris2024expandedphenotypicspectrum pages 3-4, bryant2020histoneh3.3beyond pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)	H3F3B-specific and combined	Autosomal dominant; de novo germline	Penetrance and recurrence risk are not well quantified; inherited cases appear rare
Variant spectrum	H3F3B cases include missense and stop-loss/stop-altering variants; Bryant 2020 reported 13 H3F3B patients with 12 unique H3F3B variants, and Okur 2021 added 6 H3F3B individuals including a stop-loss variant (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3)	H3F3B-specific	Missense variant; stop-loss variant; heterozygous germline variant	Full variant catalog and ClinVar classifications were not extracted here
Population frequency	Variants were absent from 138,632 gnomAD controls in the foundational cohort, supporting rarity (bryant2020histoneh3.3beyond pages 3-4)	Combined BLBS with included H3F3B variants	Ultra-rare Mendelian disorder	No prevalence or incidence estimates available
Core neurodevelopmental phenotype	Developmental delay/intellectual disability is a leading feature; in Okur 2021 all detailed individuals had global developmental delay with motor and speech delay (okur2021denovovariants pages 2-3), and 2024 authors identify developmental delay/intellectual disability as one of the four most common BLBS features (layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS; includes H3F3B individuals	HPO term suggestion: developmental delay; intellectual disability; motor delay; speech delay	Exact H3F3B-only frequencies not available in retrieved excerpts
Tone/motor phenotype	Hypotonia and broader tonal abnormalities are common; gait difficulty/ataxia and progressive gait ataxia have been described (okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS with H3F3B representation	HPO term suggestion: hypotonia; hypertonia; gait ataxia; gait disturbance	Progression appears variable and is not quantified specifically for type 2
Craniofacial phenotype	Craniofacial anomalies are a major syndrome component; in the expanded cohort, craniofacial anomalies were reported in 86% of individuals with H3-3B variants versus 95% with H3-3A variants (layocarris2024expandedphenotypicspectrum pages 3-4)	H3F3B-specific comparison available	HPO term suggestion: abnormal facial shape; dysmorphic facies; midface hypoplasia	Specific facial feature frequencies for type 2 were not fully extracted
Growth phenotype	Short stature/failure to thrive/abnormal growth are common; Okur 2021 found short stature with failure to thrive in 80%, while the expanded cohort noted 31% undergrowth/overgrowth trajectories for H3-3B variants (okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 3-4)	Mixed, partly H3F3B-specific	HPO term suggestion: short stature; failure to thrive; abnormality of head size	Growth direction and severity vary by cohort and variant
Brain and imaging phenotype	Structural brain abnormalities are common, including diminished white matter, hypomyelination, cortical dysplasia, leukoencephalopathy, cortical atrophy, and other MRI anomalies (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS with H3F3B representation	HPO term suggestion: abnormal brain MRI; white matter abnormality; hypomyelination; cortical dysplasia; cerebral atrophy	H3F3B-only imaging frequencies were not provided in the retrieved excerpts
Vision/hearing/other systems	Visual impairment and ophthalmologic problems are repeatedly reported; hearing, cardiac, endocrine, and GU anomalies are less consistently supported in later aggregate phenotyping (lubin2025couplingdeepphenotypic pages 9-11, okur2021denovovariants pages 2-3)	Combined BLBS	HPO term suggestion: visual impairment; hearing impairment; congenital hypothyroidism	Some features may reflect ascertainment or incomplete response rates rather than robust syndrome associations
Natural history	Disease course spans neurodevelopmental impairment with possible neurodegenerative progression in a subset; Bryant 2020 reported neurologic degeneration in 21% overall and 3 deaths in a 46-person mixed cohort, while Layo-Carris 2024 emphasizes phenotypic heterogeneity and uncertainty about progression (bryant2020histoneh3.3beyond pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS	Neurodevelopmental disorder; neurodegenerative course in subset	Type-2-specific mortality, survival, and longitudinal staging remain undefined
Mechanism: chromatin/protein	Germline H3.3 mutations disrupt DNA, histone, and chaperone interactions; modeling mapped many variants to nucleosome interfaces or regulator-binding regions, supporting altered chromatin function distinct from oncogenic somatic histone mutations (bryant2020histoneh3.3beyond pages 3-4, bryant2020histoneh3.3beyond pages 7-8)	Combined BLBS with H3F3B mutant lines included	GO term suggestion: chromatin organization; nucleosome assembly; protein-DNA complex assembly	Most mechanistic work is across mixed H3F3A/H3F3B variants rather than isolated H3F3B/type 2
Mechanism: epigenetic/transcriptional	Patient PTM analysis showed aberrant local mutant-histone PTM patterns, RNA-seq showed upregulated mitosis/cell-division programs, and fibroblasts showed increased proliferation with altered S/G2 phases (bryant2020histoneh3.3beyond pages 7-8)	Combined BLBS with H3F3B patient fibroblasts included	GO term suggestion: regulation of cell cycle; mitotic cell cycle; histone modification; transcriptional regulation by chromatin	Direct causal links from these molecular findings to each clinical phenotype remain incomplete
Cell/tissue involvement	Evidence points especially to nervous system development, neural crest-derived tissues, and glial biology; zebrafish studies showed craniofacial anomalies and Foxd3-derived glial defects (bryant2020histoneh3.3beyond pages 7-8)	Model-organism evidence for mixed H3.3 biology relevant to BLBS	CL term suggestion: glial cell; neural crest cell; UBERON term suggestion: brain; craniofacial skeleton	No dedicated H3F3B-only animal model was identified in retrieved evidence
Diagnostic approach	Diagnosis is currently gene-first: exome/genome sequencing or neurodevelopmental disorder panels detecting heterozygous H3F3B variants, interpreted with phenotype correlation and often brain MRI (okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)	H3F3B-specific gene, general rare-disease workflow	Molecular genetic testing; H3F3B sequencing; exome sequencing; genome sequencing	No formal consensus diagnostic criteria or biomarker assays were identified
Differential diagnosis	Authors note phenotypic overlap with leukodystrophy and other neurodevelopmental disorders with structural brain abnormalities, making genomic testing important for distinction (layocarris2024expandedphenotypicspectrum pages 8-9, okur2021denovovariants pages 2-3)	Combined BLBS	Differential term suggestion: leukodystrophy; syndromic neurodevelopmental disorder	Differential diagnosis list is not standardized in the retrieved sources
Management	No disease-modifying therapy identified; management is supportive and multidisciplinary, focused on developmental therapies, neurology follow-up, feeding/growth support, ophthalmology, and surveillance guided by symptoms and imaging (layocarris2024expandedphenotypicspectrum pages 8-9, okur2021denovovariants pages 2-3)	Combined BLBS applied to type 2	NCIT/clinical intervention term suggestion: supportive care; physical therapy; occupational therapy; speech therapy; genetic counseling	Published treatment-outcome data are extremely sparse
Prevention/counseling	Primary prevention is not established; useful measures are genetic counseling, trio testing to confirm de novo status, recurrence-risk discussion, and consideration of prenatal/preimplantation testing once a familial variant is known (layocarris2024expandedphenotypicspectrum pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)	H3F3B-relevant Mendelian counseling principles	Genetic counseling; prenatal diagnosis; preimplantation genetic testing	Empiric recurrence risk and mosaicism data are not well defined
Epidemiology	Ultra-rare disorder with no population incidence/prevalence estimate located; evidence comes from aggregated case series and matchmaker-style ascertainment rather than registries (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS	Rare disease; case-series evidence	Strong ascertainment bias; no denominator-based epidemiology
Prognosis	Prognosis is variable and incompletely defined, ranging from severe developmental disability to later-onset progressive neurologic manifestations; survival data are inadequate for type 2 specifically (bryant2020histoneh3.3beyond pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)	Combined BLBS	Variable expressivity	No validated prognostic biomarkers or genotype-specific outcome models
Model systems	Functional evidence includes patient fibroblasts and zebrafish; fibroblasts showed cell-cycle/proliferation abnormalities, and zebrafish showed craniofacial and glial phenotypes (bryant2020histoneh3.3beyond pages 7-8)	Combined BLBS/mechanistic	Patient-derived fibroblast model; zebrafish model	No dedicated mammalian H3F3B BLBNS2 model identified in retrieved evidence

Table: This table summarizes the best-supported current knowledge for Bryant-Li-Bhoj neurodevelopmental syndrome 2, emphasizing what is specific to H3F3B versus what is only available from combined BLBS cohorts. It is useful as a compact knowledge-base scaffold and highlights key evidence gaps.

1. Disease information

Definition and identifiers

BLBNS2 is a congenital/pediatric-onset neurodevelopmental disorder—a Mendelian “histonopathy”—caused by germline variation in H3-3B/H3F3B, which encodes histone H3.3. Open Targets maps the disease to MONDO:0030607, OMIM 619721, and Ensembl target ENSG00000132475. The disease became OMIM-classified in 2022. The broader BLBS designation covers both OMIM 619720 and 619721. (layocarris2024expandedphenotypicspectrum pages 3-4, OpenTargets Search: Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B)

Preferred name: Bryant-Li-Bhoj neurodevelopmental syndrome 2.
Synonyms/related labels: BLBNS2; H3F3B-related Bryant-Li-Bhoj syndrome; H3-3B-related neurodevelopmental disorder; histone H3.3-related chromatinopathy, H3-3B subtype. “Bryant-Li-Bhoj syndrome” or “BLBS” is broader and should not be used as an exact synonym when gene-specific precision is required.

No dedicated ICD-10, ICD-11, or MeSH code was identified. In clinical coding, nonspecific codes for intellectual disability, developmental disorder, hypotonia, ataxia, or congenital malformations may therefore be used, but these are not disease identifiers.

Evidence provenance

Knowledge derives primarily from aggregated, investigator-assembled cohorts identified through clinical exome sequencing, Matchmaker Exchange/GeneMatcher-like networks, and literature review—not from population registries or routine EHR-derived epidemiology. The foundational 2020 study included 46 individuals with H3-3A/H3-3B disease, including 13 H3-3B patients carrying 12 unique H3-3B variants. A 2021 cohort added six H3-3B individuals, while the 2024 expanded BLBS analysis reached 96 combined cases. (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

2. Etiology, risk, protection, and gene–environment relationships

Causal factor

The primary cause is a heterozygous germline H3-3B variant that alters histone H3.3 sequence, abundance, nucleosome interactions, chaperone binding, or post-translational regulation. Most established variants are missense; stop-loss and transcript-dependent stop-gain consequences have also been reported. The original variants were absent from 138,632 gnomAD controls, consistent with strong rarity and pathogenic constraint. (layocarris2024expandedphenotypicspectrum pages 3-4, bryant2020histoneh3.3beyond pages 3-4)

The original paper’s abstract states: “Germ line mutations in H3F3A and H3F3B cause a previously unidentified neurodevelopmental syndrome.” It further describes “46 patients bearing de novo germline mutations … with progressive neurologic dysfunction and congenital anomalies without malignancies.” This is human cohort evidence, not merely computational inference. Bryant et al., Science Advances, 4 December 2020, PMID 33268356, DOI/URL: https://doi.org/10.1126/sciadv.abc9207. (bryant2020histoneh3.3beyond pages 3-4, bryant2020histoneh3.3beyond pages 2-3)

Risk factors

Genetic: a pathogenic/likely pathogenic H3-3B allele is the dominant risk factor. Most cases are de novo, but a maternally inherited p.Asn108Ser (p.N108S) allele demonstrates that transmission can occur. Variant position and whether H3-3A or H3-3B is affected appear to influence phenotype more than sex, but robust genotype–phenotype rules have not been established. (layocarris2024expandedphenotypicspectrum pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)
Environmental, lifestyle, infectious, occupational, age, or sex-related risk: none established.
Family history: usually absent because most variants are de novo; a positive family history is possible in inherited disease or parental mosaicism.
Modifier genes: none validated. The striking variability among people with the same allele implies additional genetic, epigenetic, developmental, or environmental modifiers, but these remain hypotheses. (layocarris2024expandedphenotypicspectrum pages 3-4)

Protective factors and gene–environment interaction

No protective allele, diet, behavior, exposure, medication, or environmental intervention has been shown to prevent or attenuate BLBNS2. Histone H3.3 lies at the genetics–epigenetics interface, so gene–environment interactions are biologically plausible, but no reproducible interaction has been demonstrated in patients. The 2024 authors explicitly concluded that unmeasured factors likely modify expressivity and highlighted gene–environment research as a future priority. (layocarris2024expandedphenotypicspectrum pages 3-4)

3. Phenotypes

Core clinical spectrum

Phenotype	Type and usual timing/course	Best available frequency	Suggested HPO annotation
Developmental delay/intellectual disability	Sign; infancy/early childhood; mild to severe, generally chronic	All 10 deeply phenotyped H3-3A/H3-3B patients in Okur et al. had global motor and speech delay; exact type-2 rate unavailable	Global developmental delay; Intellectual disability; Delayed motor development; Speech delay
Hypotonia/hypertonia	Sign; often infancy; variable and sometimes mixed over time	Hypotonia 80% in the 10-person pooled 2021 cohort; tone abnormality is one of the four leading features in the 2024 cohort	Hypotonia; Hypertonia; Abnormal muscle tone
Gait impairment/ataxia	Sign; childhood onward; may be progressive	Gait difficulty 70% in the pooled 2021 cohort; progressive gait ataxia reported in later BLBS cases	Gait disturbance; Gait ataxia; Progressive ataxia
Craniofacial dysmorphism	Physical manifestation; congenital/stable	86% in H3-3B versus 95% in H3-3A in the expanded cohort	Abnormal facial shape; Facial asymmetry; Midface hypoplasia; Thin upper lip vermilion
Abnormal growth/failure to thrive	Sign; infancy/childhood; variable under- or overgrowth	Short stature/failure to thrive 80% in pooled 2021 cases; 31% of H3-3B cases had undergrowth/overgrowth trajectories in the expanded analysis	Failure to thrive; Short stature; Abnormal body height; Abnormal head size
Microcephaly	Physical sign; congenital or acquired	26% in the original 46-person combined cohort; 60% in the 2021 ten-person cohort	Microcephaly; Acquired microcephaly
Brain MRI abnormality	Imaging sign; may evolve	Cortical atrophy 26% in the original combined cohort; structural abnormalities 57% among evaluable 2021 cases	Abnormal brain MRI; Cerebral atrophy; White matter abnormality; Hypomyelination; Cortical dysplasia
Visual impairment	Symptom/sign; pediatric	Recurrent feature; exact H3-3B rate unavailable	Visual impairment; Abnormality of the eye
Skeletal/extremity anomalies	Physical manifestation; usually congenital	Reported recurrently but incompletely quantified	Abnormality of the musculoskeletal system; Abnormality of the hand/foot
Regression/neurodegeneration	Sign/course; childhood through adulthood; subset only	Neurologic degeneration 21% in the original combined cohort	Developmental regression; Neurodegeneration

The quantitative values above must not be interpreted as population prevalence estimates. H3-3B-specific data are available for craniofacial findings and growth trajectory, whereas most other percentages pool both genes. (layocarris2024expandedphenotypicspectrum pages 3-4, bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

Brain abnormalities reported across BLBS include diminished white matter, hypomyelination, leukodystrophy-like change, cortical dysplasia, leukoencephalopathy, and cerebral/cortical atrophy. Repeat MRI may help distinguish static developmental anomalies from progressive white-matter or volume loss. (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

Variability, function, and quality of life

Severity ranges from mild delay to profound disability with inability to attain independent ambulation. In the original combined cohort, 73% attained independent sitting; recurrent variants could nevertheless produce markedly different milestones, regression, seizures, and tone findings. Such variability argues against deterministic prognosis based only on variant identity. (bryant2020histoneh3.3beyond pages 3-4, bryant2020histoneh3.3beyond pages 2-3)

No BLBNS2-specific EQ-5D, SF-36, PROMIS, caregiver-burden, or other quality-of-life study was found. Expected effects include impaired communication, mobility, feeding, education, self-care, and independent living, but quantitative utility weights are unavailable.

Seizures, hearing abnormalities, cardiac/circulatory findings, genitourinary findings, and endocrine abnormalities occur in some reports, but a 2025 deep-phenotyping analysis did not support them as consistently associated BLBS features. They should be recorded and treated when present without being considered obligatory manifestations. (lubin2025couplingdeepphenotypic pages 9-11)

4. Genetic and molecular information

Gene and variant classes

Causal gene: H3-3B/H3F3B, encoding histone H3.3B. The original 46-person study found 13 H3-3B cases and 12 unique H3-3B variants; a recurrent H3-3B allele was p.Pro121Arg. More recent examples include p.Gly34Arg, p.Gly34Val, p.Asn108Ser, p.Val117Val/p.Ser147Ter (transcript-dependent consequence), and a two-base stop-loss deletion reported as p.137Cext9 or p.Cys136*ext9, depending on transcript/notation. (layocarris2024expandedphenotypicspectrum pages 3-4, bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

Variant classes: predominantly missense, but synonymous variants with noncanonical-transcript stop consequences and stop-loss/extension variants establish a broader allelic spectrum. Most are germline and heterozygous. No evidence supports a somatic mosaic tumor mechanism as the usual cause of BLBNS2.

Classification: pathogenicity requires ACMG/AMP assessment integrating de novo occurrence, population absence, phenotype consistency, recurrence, structural location, and functional data. A variant should not be labeled pathogenic solely because it alters H3-3B; variant-level ClinVar assertions may differ and require current review.

Population frequency: established cohort variants were absent from 138,632 gnomAD controls. Exact per-variant gnomAD v4, TOPMed, and ancestry-stratified frequencies were not available in the retrieved evidence. (bryant2020histoneh3.3beyond pages 3-4)

Functional effects

Variant consequences are heterogeneous rather than a single simple null mechanism. Structural modeling of 37 variants at 25 H3.3 residues predicted disruption of DNA contacts, histone-octamer stability, intramolecular contacts, or binding to histone chaperones and epigenetic regulators. Experimental constructs showed reduced abundance for p.Arg129His, p.Met121Ile, and p.Ile52Asn, increased abundance for p.Arg41Cys, and approximately wild-type abundance for an elongated stop-loss protein. p.Arg129His increased interaction with the chaperone DAXX in one assay. These observations support altered-function, stability, and interaction mechanisms rather than uniform haploinsufficiency. (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3)

No validated modifier gene, disease-specific DNA-methylation episignature, large chromosomal rearrangement, recurrent CNV, or parent-of-origin effect is established. Conventional aneuploidy, translocation, and repeat-expansion mechanisms are not characteristic.

5. Environmental information

No toxin, radiation exposure, pollutant, occupation, smoking, alcohol use, diet, physical activity pattern, infection, or microbiome state has been shown to cause or trigger BLBNS2. It is not infectious or contagious. Environmental history remains clinically useful for excluding phenocopies and managing general health, but it does not presently alter molecular diagnosis.

6. Mechanism and pathophysiology

Proposed causal chain

Upstream trigger: a heterozygous germline H3-3B sequence variant produces an altered H3.3 molecule.
Nucleosome-level dysfunction: the mutant histone changes DNA contacts, nucleosome stability, or interaction with chaperones such as DAXX/UBN1 and chromatin readers.
Epigenetic disturbance: post-translational modifications are altered locally in cis on mutant H3.3 rather than globally on all histones, distinguishing these germline alleles from canonical somatic “oncohistone” mechanisms.
Transcription/cell-cycle disturbance: patient-cell RNA sequencing shows upregulation of mitosis and cell-division programs; fibroblasts demonstrate increased proliferation and altered S/G2 distribution without a major viability defect.
Developmental tissue effects: disturbed chromatin regulation affects neural development, glial/neural-crest derivatives, craniofacial development, and potentially white-matter maintenance.
Clinical outcome: developmental delay, intellectual disability, tone and gait abnormalities, dysmorphism, growth disturbance, and—in a subset—progressive neurologic dysfunction or atrophy. (bryant2020histoneh3.3beyond pages 3-4, bryant2020histoneh3.3beyond pages 7-8)

In five patient fibroblast lines, including lines with H3-3B p.Gly34Val, p.Val117Val, and p.Ser146Ter-related variants, proliferation was increased relative to six controls, with significant S- and G2-phase changes. This is direct patient-derived in-vitro evidence but does not prove that excess proliferation is the sole neuronal disease mechanism. (bryant2020histoneh3.3beyond pages 7-8)

The foundational abstract reports: “Patient histone posttranslational modifications analysis revealed notably aberrant local PTM patterns” and “RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division.” It concludes that germline and cancer-associated mutations are mechanistically distinct but may converge on proliferation control. (bryant2020histoneh3.3beyond pages 7-8)

Relevant ontology suggestions

GO biological process: chromatin organization; nucleosome assembly; regulation of transcription by chromatin organization; histone modification; mitotic cell cycle; regulation of cell proliferation; nervous-system development; glial-cell development; neural-crest-cell development.
GO cellular component: nucleosome; chromatin; nucleus; chromosome.
CL cell types: neuron; glial cell; neural crest cell; oligodendrocyte lineage cell; fibroblast for patient-cell assays.
Molecular profiling: disease-relevant bulk RNA sequencing, histone PTM proteomics, interaction proteomics, and cell-cycle profiling exist. No validated patient single-cell, spatial transcriptomic, metabolomic, lipidomic, or integrated clinical multi-omic signature was identified.
Immune/metabolic/tissue-injury mechanisms: no primary immunodeficiency, autoimmunity, enzyme deficiency, mitochondrial metabolic defect, fibrosis, ischemia, or inflammatory tissue-injury pathway is established.

7. Anatomical structures affected

Primary system: central nervous system, particularly brain development and white matter. Suggested terms include UBERON:brain, cerebral cortex, cerebral white matter, cerebellum, and central nervous system. MRI abnormalities are generally bilateral/diffuse rather than a consistently lateralized lesion. (bryant2020histoneh3.3beyond pages 3-4, okur2021denovovariants pages 2-3)

Secondary structures: craniofacial skeleton and soft tissues, eyes/visual system, skeletal/extremity structures, and gastrointestinal/feeding systems. Cardiac, endocrine, auditory, and genitourinary abnormalities may occur but are not firmly established as core type-2 manifestations. (lubin2025couplingdeepphenotypic pages 9-11, okur2021denovovariants pages 2-3)

Cellular/subcellular localization: neurons and glia are clinically implicated; neural-crest derivatives are supported by zebrafish data. The fundamental subcellular site is nuclear chromatin/nucleosomes rather than mitochondria, lysosome, or endoplasmic reticulum. Suggested GO cellular-component terms are nucleus, chromatin, chromosome, and nucleosome.

8. Temporal development and natural history

BLBNS2 is usually congenital or early pediatric in onset, although the molecular lesion is present from conception. Developmental delay, hypotonia, feeding/growth concerns, or dysmorphism commonly become evident in infancy or early childhood. The course is lifelong and highly variable.

The original mixed cohort, aged 2 months to 32 years, documented neurologic degeneration in 9/46 (21%), cortical atrophy in 26%, and three deaths. These figures demonstrate that progression and mortality can occur but do not establish H3-3B-specific survival risks. Later work emphasizes a mixed neurodevelopmental–neurodegenerative spectrum, including progressive gait ataxia, rather than universal degeneration. (bryant2020histoneh3.3beyond pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)

There are no validated stages, median progression rate, remission pattern, or critical therapeutic window. Developmental surveillance and early habilitation are rational because infancy and early childhood are periods of high neuroplasticity, but no study has quantified a disease-specific window of reversibility.

9. Inheritance and population

Inheritance

The best model is autosomal dominant, usually due to a de novo heterozygous germline allele. One maternally inherited p.Asn108Ser variant expands the spectrum beyond strictly de novo disease. (bryant2020histoneh3.3beyond pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)

Penetrance is unknown; expressivity is clearly variable, including among individuals with the same allele. No anticipation, founder effect, consanguinity association, carrier frequency, or population-specific enrichment has been established. Parental germline or low-level somatic mosaicism has not been quantified and remains relevant to recurrence counseling.

Epidemiology

No incidence, prevalence, sex ratio, ethnic predisposition, or geographic distribution can be estimated reliably. Published cases span multiple countries and ancestries, supporting worldwide occurrence rather than endemicity. Case discovery depends heavily on access to exome/genome sequencing. The original combined cohort contained 27 males and 19 females, but this ascertainment ratio is not evidence of sex-biased risk. (bryant2020histoneh3.3beyond pages 3-4)

10. Diagnostics

Recommended approach

Clinical recognition: unexplained developmental/intellectual disability with tone abnormality, gait disorder, growth disturbance, dysmorphism, visual problems, or abnormal brain MRI.
First-line molecular testing: trio whole-exome or whole-genome sequencing is preferred for a genetically heterogeneous neurodevelopmental presentation. A comprehensive neurodevelopmental/chromatinopathy panel should include H3-3B/H3F3B and H3-3A/H3F3A.
Variant confirmation: orthogonal confirmation as appropriate, parental testing, transcript-aware HGVS annotation, population-database review, and ACMG/AMP classification.
Phenotypic assessment: developmental evaluation; neurologic examination; growth and feeding assessment; ophthalmology; hearing testing; and brain MRI. EEG is indicated for suspected seizures or episodic movements, not universally as a diagnostic biomarker.
Longitudinal evaluation: repeat neurologic, gait, vision, growth, and developmental assessment; repeat MRI when progression, regression, ataxia, or new focal findings emerge. (okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

The 2021 abstract states that detailed phenotyping of H3-3A and H3-3B cases showed “global developmental delay, short stature, failure to thrive, dysmorphic facial features, structural brain abnormalities, hypotonia, and visual impairment.” Okur et al., npj Genomic Medicine, December 2021, PMID 34876591, DOI/URL: https://doi.org/10.1038/s41525-021-00268-8. (okur2021denovovariants pages 2-3)

Role of other tests

CMA: useful when first-tier testing for developmental disability or to find an alternative CNV, but cannot reliably detect most causative H3-3B single-nucleotide variants.
Karyotype/FISH: not routinely informative unless another chromosomal diagnosis is suspected.
Single-gene sequencing: reasonable for a highly suggestive phenotype or familial variant, but exome/genome testing usually offers better differential coverage.
RNA sequencing: potentially useful for transcript-dependent splice/stop consequences or uncertain variants, but not a validated routine assay.
Methylation/proteomics/metabolomics: no clinically validated BLBNS2 signature.
Mitochondrial DNA and repeat-expansion tests: not disease-specific; use only when the differential warrants them.

Differential diagnosis

Important alternatives include H3-3A-related BLBNS1, other germline histonopathies/chromatinopathies, leukodystrophies and hypomyelinating disorders, cerebral-palsy phenocopies, mitochondrial disease, congenital disorders of glycosylation, syndromic intellectual disability with growth failure, and genetic ataxia or movement disorders. Leukodystrophy overlap is especially relevant when progressive gait ataxia or white-matter abnormalities dominate. (okur2021denovovariants pages 2-3, layocarris2024expandedphenotypicspectrum pages 8-9)

There are no standardized clinical diagnostic criteria, prenatal ultrasound signature, newborn biochemical screen, or population screening program.

11. Outcome and prognosis

Prognosis is variable. Developmental impairment is generally lifelong; functional outcomes range from delayed but achieved milestones to severe dependence. Regression, progressive ataxia, and cerebral atrophy occur in a subset. Three deaths were reported in the original 46-person combined cohort, but causes, age-specific mortality, and H3-3B-specific contribution are insufficient to estimate life expectancy or survival curves. (bryant2020histoneh3.3beyond pages 3-4)

No validated prognostic biomarker exists. Variant location and affected paralog may influence particular phenotypes, but neither currently supports an individual outcome calculator. Sex was less explanatory than gene and protein position in the expanded analysis, while substantial residual variability remained. (layocarris2024expandedphenotypicspectrum pages 3-4)

Potential complications include feeding/growth failure, immobility, contracture or orthopedic problems, communication impairment, falls from ataxia, visual disability, seizures in a minority, and caregiver burden. No formal recovery rate or treatment-response rate has been published.

12. Treatment and current applications

Current clinical implementation

There is no FDA/EMA-approved disease-modifying or genotype-targeted treatment. Care is multidisciplinary and phenotype-directed:

developmental pediatrics and neurology follow-up;
early physical, occupational, speech/language, communication, and feeding therapy;
nutritional support and swallowing assessment when indicated;
mobility aids, orthotics, and orthopedic management for gait or skeletal problems;
standard antiseizure medication when epilepsy is documented;
ophthalmologic correction or low-vision support;
audiology and endocrine/cardiac evaluation when clinically indicated;
educational planning, augmentative communication, psychosocial support, respite care, and genetic counseling.

Suggested NCIT intervention concepts include Supportive Care, Physical Therapy, Occupational Therapy, Speech Therapy, Nutritional Support, Genetic Counseling, and Anticonvulsant Therapy. These are ontology suggestions, not BLBNS2-specific efficacy claims.

No BLBNS2 interventional clinical trial was identified. Trials retrieved through broad H3F3A/H3F3B searches concerned somatic H3-mutant gliomas and must not be extrapolated to germline BLBNS2. Gene replacement, CRISPR editing, allele-selective silencing, ASOs, cell therapy, immunotherapy, and chromatin-targeted drugs remain preclinical concepts without demonstrated patient benefit.

13. Prevention

Primary prevention: no lifestyle, vaccine, environmental, or pharmacologic intervention prevents a de novo germline H3-3B variant.

Secondary prevention/early detection: prompt genomic testing can shorten the diagnostic odyssey and initiate developmental, feeding, vision, mobility, and educational support. There is no newborn screening assay or evidence supporting population screening.

Tertiary prevention: monitor growth, swallowing, mobility, contractures, vision, neurologic regression, and seizures to reduce avoidable complications.

Reproductive options: genetic counseling; parental testing; assessment for parental mosaicism when technically feasible; prenatal diagnosis by CVS/amniocentesis; or preimplantation genetic testing once a familial pathogenic variant is established. For an affected heterozygous individual, theoretical transmission risk is 50% per pregnancy, subject to penetrance and reproductive fitness. After an apparently de novo case, recurrence risk is low but not zero because germline mosaicism cannot be excluded. No disease-specific empirical mosaicism estimate is available.

14. Other species and natural disease

No naturally occurring veterinary counterpart, affected breed, zoonotic reservoir, or cross-species transmission phenomenon was identified. BLBNS2 is genetic and noncommunicable. H3.3 biology is evolutionarily conserved, making animal orthologs useful experimentally, but conservation does not establish spontaneous animal disease.

Relevant experimental taxa include Homo sapiens (NCBI Taxon 9606), Danio rerio (zebrafish; Taxon 7955), and potentially Mus musculus (mouse; Taxon 10090). Exact ortholog NCBI Gene identifiers should be verified directly before database ingestion.

15. Model organisms and experimental systems

Patient-derived cells

Patient fibroblasts are the strongest disease-proximal in-vitro model. They demonstrate variant-specific PTM changes, altered transcriptional programs, increased proliferation, and S/G2 cell-cycle changes. Their limitation is that fibroblasts do not reproduce neuronal/glial maturation, circuitry, myelination, or neurodegeneration. (bryant2020histoneh3.3beyond pages 7-8)

Zebrafish

H3.3 perturbation in zebrafish produced craniofacial anomalies and defects in Foxd3-derived glia and other neural-crest derivatives, providing organism-level support for craniofacial and glial mechanisms. The original abstract states: “A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia.” However, this is not a fully validated H3-3B-variant-specific model of every human phenotype. (bryant2020histoneh3.3beyond pages 7-8)

Other models and applications

Structural modeling and transfected variant constructs have been used to examine nucleosome interfaces, protein abundance, and chaperone binding. No dedicated mammalian knock-in model, patient iPSC-derived neural model, cerebral organoid, or H3-3B-specific longitudinal vertebrate model was identified in the retrieved evidence. Priority applications include testing allele-specific mechanisms, defining neuronal versus oligodendroglial vulnerability, studying myelination and regression, and evaluating allele-selective therapeutic strategies.

Recent developments and expert interpretation

The major 2024 advance was expansion from an initially narrow “de novo missense neurodegenerative syndrome” to a 96-person, phenotypically heterogeneous neurodevelopmental–neurodegenerative spectrum encompassing missense, synonymous/transcript-dependent, stop-loss, and inherited alleles. The authors found no conclusive simple genotype–phenotype correlation; gene and protein location appeared more informative than sex, but substantial variability remained. Layo-Carris et al., European Journal of Human Genetics 32:928–937, published April 2024, DOI/URL: https://doi.org/10.1038/s41431-024-01610-1. (layocarris2024expandedphenotypicspectrum pages 3-4, layocarris2024expandedphenotypicspectrum pages 8-9)

The paper’s abstract summarizes the current expert position: “phenotypic heterogeneity was present even amongst individuals harboring the same variant” and “additional factors may play a role in modifying the phenotypes.” This supports cautious individualized prognosis and continued longitudinal study rather than rigid variant-based prediction. (layocarris2024expandedphenotypicspectrum pages 3-4)

A 2025 deep-phenotyping study of 192 individuals with germline histonopathies further reinforced developmental delay/intellectual disability, motor and speech delay, MRI abnormalities, hypotonia, vision abnormalities, head-size abnormalities, and skeletal findings as central BLBS domains, while finding weaker support for seizures and several non-neurologic systems as consistent syndrome associations. It also reported an adult H3-3B p.Lys23Arg carrier with an atypical pulmonary carcinoid at age 33; this isolated observation is insufficient to establish cancer predisposition or justify routine tumor surveillance. Lubin et al., Human Genetics and Genomics Advances 6:100440, July 2025, https://doi.org/10.1016/j.xhgg.2025.100440. (lubin2025couplingdeepphenotypic pages 9-11)

Principal knowledge gaps

H3-3B-specific phenotype denominators and longitudinal natural history.
Incidence, prevalence, penetrance, sex ratio, mortality, and life expectancy.
Systematic parental-mosaicism and reproductive-risk data.
Variant-level functional classification and reproducible genotype–phenotype relationships.
Neuron-, oligodendrocyte-, and brain-organoid models specific to H3-3B alleles.
Validated epigenomic, transcriptomic, imaging, or fluid biomarkers.
Controlled evidence for surveillance and supportive interventions.
Disease-modifying therapies and BLBNS2-specific clinical trials.

Accordingly, BLBNS2 should currently be represented in a knowledge base as a de novo-dominant, variably expressive H3-3B histonopathy with developmental and possible progressive neurologic manifestations, while marking most frequency, prognosis, and treatment fields as incomplete rather than absent.

References

(layocarris2024expandedphenotypicspectrum pages 3-4): Dana E. Layo-Carris, Emily E. Lubin, Annabel K. Sangree, Kelly J. Clark, Emily L. Durham, Elizabeth M. Gonzalez, Sarina Smith, Rajesh Angireddy, Xiao Min Wang, Erin Weiss, Annick Toutain, Roberto Mendoza-Londono, Lucie Dupuis, Nadirah Damseh, Danita Velasco, Irene Valenzuela, Marta Codina-Solà, Catherine Ziats, Jaclyn Have, Katie Clarkson, Dora Steel, Manju Kurian, Katy Barwick, Diana Carrasco, Aditi I. Dagli, M. J. M. Nowaczyk, Miroslava Hančárová, Šárka Bendová, Darina Prchalova, Zdeněk Sedláček, Alica Baxová, Catherine Bearce Nowak, Jessica Douglas, Wendy K. Chung, Nicola Longo, Konrad Platzer, Chiara Klöckner, Luisa Averdunk, Dagmar Wieczorek, Ilona Krey, Christiane Zweier, Andre Reis, Tugce Balci, Marleen Simon, Hester Y. Kroes, Antje Wiesener, Georgia Vasileiou, Nikolaos M. Marinakis, Danai Veltra, Christalena Sofocleous, Konstantina Kosma, Joanne Traeger Synodinos, Konstantinos A. Voudris, Marie-Laure Vuillaume, Paul Gueguen, Nicolas Derive, Estelle Colin, Clarisse Battault, Billie Au, Martin Delatycki, Mathew Wallis, Lyndon Gallacher, Fatma Majdoub, Noor Smal, Sarah Weckhuysen, An-Sofie Schoonjans, R. Frank Kooy, Marije Meuwissen, Benjamin T. Cocanougher, Kathryn Taylor, Carolyn E. Pizoli, Marie T. McDonald, Philip James, Elizabeth R. Roeder, Rebecca Littlejohn, Nicholas A. Borja, Willa Thorson, Kristine King, Radka Stoeva, Manon Suerink, Esther Nibbeling, Stephanie Baskin, Gwenaël L. E. Guyader, Julie Kaplan, Candace Muss, Deanna Alexis Carere, Elizabeth J. K. Bhoj, and Laura M. Bryant. Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder bryant-li-bhoj syndrome with 38 additional individuals. European Journal of Human Genetics, 32:928-937, Apr 2024. URL: https://doi.org/10.1038/s41431-024-01610-1, doi:10.1038/s41431-024-01610-1. This article has 12 citations and is from a domain leading peer-reviewed journal.
(OpenTargets Search: Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B): Open Targets Query (Bryant-Li-Bhoj neurodevelopmental syndrome 2-H3F3B, 2 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(okur2021denovovariants pages 2-3): Volkan Okur, Zefu Chen, Liesbeth Vossaert, Sandra Peacock, Jill Rosenfeld, Lina Zhao, Haowei Du, Emily Calamaro, Amanda Gerard, Sen Zhao, Jill Kelsay, Ashley Lahr, Chloe Mighton, Hillary M. Porter, Amy Siemon, Josh Silver, Shayna Svihovec, Chin-To Fong, Christina L. Grant, Jordan Lerner-Ellis, Kandamurugu Manickam, Suneeta Madan-Khetarpal, Shawn E. McCandless, Chantal F. Morel, G. Bradley Schaefer, Elizabeth M. Berry-Kravis, Ryan Gates, Natalia Gomez-Ospina, Guixing Qiu, Terry Jianguo Zhang, Zhihong Wu, Linyan Meng, Pengfei Liu, Daryl A. Scott, James R. Lupski, Christine M. Eng, Nan Wu, and Bo Yuan. De novo variants in h3-3a and h3-3b are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities. npj Genomic Medicine, Dec 2021. URL: https://doi.org/10.1038/s41525-021-00268-8, doi:10.1038/s41525-021-00268-8. This article has 32 citations and is from a peer-reviewed journal.
(bryant2020histoneh3.3beyond pages 3-4): Laura Bryant, Dong Li, Samuel G. Cox, Dylan Marchione, Evan F. Joiner, Khadija Wilson, Kevin Janssen, Pearl Lee, Michael E. March, Divya Nair, Elliott Sherr, Brieana Fregeau, Klaas J. Wierenga, Alexandrea Wadley, Grazia M. S. Mancini, Nina Powell-Hamilton, Jiddeke van de Kamp, Theresa Grebe, John Dean, Alison Ross, Heather P. Crawford, Zoe Powis, Megan T. Cho, Marcia C. Willing, Linda Manwaring, Rachel Schot, Caroline Nava, Alexandra Afenjar, Davor Lessel, Matias Wagner, Thomas Klopstock, Juliane Winkelmann, Claudia B. Catarino, Kyle Retterer, Jane L. Schuette, Jeffrey W. Innis, Amy Pizzino, Sabine Lüttgen, Jonas Denecke, Tim M. Strom, Kristin G. Monaghan, Zuo-Fei Yuan, Holly Dubbs, Renee Bend, Jennifer A. Lee, Michael J. Lyons, Julia Hoefele, Roman Günthner, Heiko Reutter, Boris Keren, Kelly Radtke, Omar Sherbini, Cameron Mrokse, Katherine L. Helbig, Sylvie Odent, Benjamin Cogne, Sandra Mercier, Stephane Bezieau, Thomas Besnard, Sebastien Kury, Richard Redon, Karit Reinson, Monica H. Wojcik, Katrin Õunap, Pilvi Ilves, A. Micheil Innes, Kristin D. Kernohan, Gregory Costain, M. Stephen Meyn, David Chitayat, Elaine Zackai, Anna Lehman, Hilary Kitson, Martin G. Martin, Julian A. Martinez-Agosto, Stan F. Nelson, Christina G. S. Palmer, Jeanette C. Papp, Neil H. Parker, Janet S. Sinsheimer, Eric Vilain, Jijun Wan, Amanda J. Yoon, Allison Zheng, Elise Brimble, Giovanni Battista Ferrero, Francesca Clementina Radio, Diana Carli, Sabina Barresi, Alfredo Brusco, Marco Tartaglia, Jennifer Muncy Thomas, Luis Umana, Marjan M. Weiss, Garrett Gotway, K. E. Stuurman, Michelle L. Thompson, Kirsty McWalter, Constance T. R. M. Stumpel, Servi J. C. Stevens, Alexander P. A. Stegmann, Kristian Tveten, Arve Vøllo, Trine Prescott, Christina Fagerberg, Lone Walentin Laulund, Martin J. Larsen, Melissa Byler, Robert Roger Lebel, Anna C. Hurst, Joy Dean, Samantha A. Schrier Vergano, Jennifer Norman, Saadet Mercimek-Andrews, Juanita Neira, Margot I. Van Allen, Nicola Longo, Elizabeth Sellars, Raymond J. Louie, Sara S. Cathey, Elly Brokamp, Delphine Heron, Molly Snyder, Adeline Vanderver, Celeste Simon, Xavier de la Cruz, Natália Padilla, J. Gage Crump, Wendy Chung, Benjamin Garcia, Hakon H. Hakonarson, and Elizabeth J. Bhoj. Histone h3.3 beyond cancer: germline mutations in histone 3 family 3a and 3b cause a previously unidentified neurodegenerative disorder in 46 patients. Science Advances, Dec 2020. URL: https://doi.org/10.1126/sciadv.abc9207, doi:10.1126/sciadv.abc9207. This article has 92 citations and is from a highest quality peer-reviewed journal.
(layocarris2024expandedphenotypicspectrum pages 8-9): Dana E. Layo-Carris, Emily E. Lubin, Annabel K. Sangree, Kelly J. Clark, Emily L. Durham, Elizabeth M. Gonzalez, Sarina Smith, Rajesh Angireddy, Xiao Min Wang, Erin Weiss, Annick Toutain, Roberto Mendoza-Londono, Lucie Dupuis, Nadirah Damseh, Danita Velasco, Irene Valenzuela, Marta Codina-Solà, Catherine Ziats, Jaclyn Have, Katie Clarkson, Dora Steel, Manju Kurian, Katy Barwick, Diana Carrasco, Aditi I. Dagli, M. J. M. Nowaczyk, Miroslava Hančárová, Šárka Bendová, Darina Prchalova, Zdeněk Sedláček, Alica Baxová, Catherine Bearce Nowak, Jessica Douglas, Wendy K. Chung, Nicola Longo, Konrad Platzer, Chiara Klöckner, Luisa Averdunk, Dagmar Wieczorek, Ilona Krey, Christiane Zweier, Andre Reis, Tugce Balci, Marleen Simon, Hester Y. Kroes, Antje Wiesener, Georgia Vasileiou, Nikolaos M. Marinakis, Danai Veltra, Christalena Sofocleous, Konstantina Kosma, Joanne Traeger Synodinos, Konstantinos A. Voudris, Marie-Laure Vuillaume, Paul Gueguen, Nicolas Derive, Estelle Colin, Clarisse Battault, Billie Au, Martin Delatycki, Mathew Wallis, Lyndon Gallacher, Fatma Majdoub, Noor Smal, Sarah Weckhuysen, An-Sofie Schoonjans, R. Frank Kooy, Marije Meuwissen, Benjamin T. Cocanougher, Kathryn Taylor, Carolyn E. Pizoli, Marie T. McDonald, Philip James, Elizabeth R. Roeder, Rebecca Littlejohn, Nicholas A. Borja, Willa Thorson, Kristine King, Radka Stoeva, Manon Suerink, Esther Nibbeling, Stephanie Baskin, Gwenaël L. E. Guyader, Julie Kaplan, Candace Muss, Deanna Alexis Carere, Elizabeth J. K. Bhoj, and Laura M. Bryant. Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder bryant-li-bhoj syndrome with 38 additional individuals. European Journal of Human Genetics, 32:928-937, Apr 2024. URL: https://doi.org/10.1038/s41431-024-01610-1, doi:10.1038/s41431-024-01610-1. This article has 12 citations and is from a domain leading peer-reviewed journal.
(lubin2025couplingdeepphenotypic pages 9-11): Emily E. Lubin, Elizabeth M. Gonzalez, Annabel K. Sangree, Emily L. Durham, Hannah Klinkhammer, Jing-Mei Li, Sarina M. Smith, Dana E. Layo-Carris, Kelly J. Clark, Ashley J. Melendez-Perez, Xiao Min Wang, Rajesh Angireddy, Erin E. Weiss, Tahsin Stefan Barakat, Sandra Mercier, Benjamin Cogné, Saskia Koene, Yvonne Hilhorst-Hofstee, Malgorzata Rydzanicz, Rafal Ploski, María de los Ángeles Gómez Cano, María Palomares-Bralo, Tania Barragán Arévalo, Tiong Yang Tan, Lyndon Gallacher, Suzanne P. MacFarland, Rebecca C. Ahrens-Nicklas, Tomoki T. Nomakuchi, and Elizabeth J.K. Bhoj. Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies. Jul 2025. URL: https://doi.org/10.1016/j.xhgg.2025.100440, doi:10.1016/j.xhgg.2025.100440. This article has 5 citations and is from a peer-reviewed journal.
(bryant2020histoneh3.3beyond pages 7-8): Laura Bryant, Dong Li, Samuel G. Cox, Dylan Marchione, Evan F. Joiner, Khadija Wilson, Kevin Janssen, Pearl Lee, Michael E. March, Divya Nair, Elliott Sherr, Brieana Fregeau, Klaas J. Wierenga, Alexandrea Wadley, Grazia M. S. Mancini, Nina Powell-Hamilton, Jiddeke van de Kamp, Theresa Grebe, John Dean, Alison Ross, Heather P. Crawford, Zoe Powis, Megan T. Cho, Marcia C. Willing, Linda Manwaring, Rachel Schot, Caroline Nava, Alexandra Afenjar, Davor Lessel, Matias Wagner, Thomas Klopstock, Juliane Winkelmann, Claudia B. Catarino, Kyle Retterer, Jane L. Schuette, Jeffrey W. Innis, Amy Pizzino, Sabine Lüttgen, Jonas Denecke, Tim M. Strom, Kristin G. Monaghan, Zuo-Fei Yuan, Holly Dubbs, Renee Bend, Jennifer A. Lee, Michael J. Lyons, Julia Hoefele, Roman Günthner, Heiko Reutter, Boris Keren, Kelly Radtke, Omar Sherbini, Cameron Mrokse, Katherine L. Helbig, Sylvie Odent, Benjamin Cogne, Sandra Mercier, Stephane Bezieau, Thomas Besnard, Sebastien Kury, Richard Redon, Karit Reinson, Monica H. Wojcik, Katrin Õunap, Pilvi Ilves, A. Micheil Innes, Kristin D. Kernohan, Gregory Costain, M. Stephen Meyn, David Chitayat, Elaine Zackai, Anna Lehman, Hilary Kitson, Martin G. Martin, Julian A. Martinez-Agosto, Stan F. Nelson, Christina G. S. Palmer, Jeanette C. Papp, Neil H. Parker, Janet S. Sinsheimer, Eric Vilain, Jijun Wan, Amanda J. Yoon, Allison Zheng, Elise Brimble, Giovanni Battista Ferrero, Francesca Clementina Radio, Diana Carli, Sabina Barresi, Alfredo Brusco, Marco Tartaglia, Jennifer Muncy Thomas, Luis Umana, Marjan M. Weiss, Garrett Gotway, K. E. Stuurman, Michelle L. Thompson, Kirsty McWalter, Constance T. R. M. Stumpel, Servi J. C. Stevens, Alexander P. A. Stegmann, Kristian Tveten, Arve Vøllo, Trine Prescott, Christina Fagerberg, Lone Walentin Laulund, Martin J. Larsen, Melissa Byler, Robert Roger Lebel, Anna C. Hurst, Joy Dean, Samantha A. Schrier Vergano, Jennifer Norman, Saadet Mercimek-Andrews, Juanita Neira, Margot I. Van Allen, Nicola Longo, Elizabeth Sellars, Raymond J. Louie, Sara S. Cathey, Elly Brokamp, Delphine Heron, Molly Snyder, Adeline Vanderver, Celeste Simon, Xavier de la Cruz, Natália Padilla, J. Gage Crump, Wendy Chung, Benjamin Garcia, Hakon H. Hakonarson, and Elizabeth J. Bhoj. Histone h3.3 beyond cancer: germline mutations in histone 3 family 3a and 3b cause a previously unidentified neurodegenerative disorder in 46 patients. Science Advances, Dec 2020. URL: https://doi.org/10.1126/sciadv.abc9207, doi:10.1126/sciadv.abc9207. This article has 92 citations and is from a highest quality peer-reviewed journal.
(bryant2020histoneh3.3beyond pages 2-3): Laura Bryant, Dong Li, Samuel G. Cox, Dylan Marchione, Evan F. Joiner, Khadija Wilson, Kevin Janssen, Pearl Lee, Michael E. March, Divya Nair, Elliott Sherr, Brieana Fregeau, Klaas J. Wierenga, Alexandrea Wadley, Grazia M. S. Mancini, Nina Powell-Hamilton, Jiddeke van de Kamp, Theresa Grebe, John Dean, Alison Ross, Heather P. Crawford, Zoe Powis, Megan T. Cho, Marcia C. Willing, Linda Manwaring, Rachel Schot, Caroline Nava, Alexandra Afenjar, Davor Lessel, Matias Wagner, Thomas Klopstock, Juliane Winkelmann, Claudia B. Catarino, Kyle Retterer, Jane L. Schuette, Jeffrey W. Innis, Amy Pizzino, Sabine Lüttgen, Jonas Denecke, Tim M. Strom, Kristin G. Monaghan, Zuo-Fei Yuan, Holly Dubbs, Renee Bend, Jennifer A. Lee, Michael J. Lyons, Julia Hoefele, Roman Günthner, Heiko Reutter, Boris Keren, Kelly Radtke, Omar Sherbini, Cameron Mrokse, Katherine L. Helbig, Sylvie Odent, Benjamin Cogne, Sandra Mercier, Stephane Bezieau, Thomas Besnard, Sebastien Kury, Richard Redon, Karit Reinson, Monica H. Wojcik, Katrin Õunap, Pilvi Ilves, A. Micheil Innes, Kristin D. Kernohan, Gregory Costain, M. Stephen Meyn, David Chitayat, Elaine Zackai, Anna Lehman, Hilary Kitson, Martin G. Martin, Julian A. Martinez-Agosto, Stan F. Nelson, Christina G. S. Palmer, Jeanette C. Papp, Neil H. Parker, Janet S. Sinsheimer, Eric Vilain, Jijun Wan, Amanda J. Yoon, Allison Zheng, Elise Brimble, Giovanni Battista Ferrero, Francesca Clementina Radio, Diana Carli, Sabina Barresi, Alfredo Brusco, Marco Tartaglia, Jennifer Muncy Thomas, Luis Umana, Marjan M. Weiss, Garrett Gotway, K. E. Stuurman, Michelle L. Thompson, Kirsty McWalter, Constance T. R. M. Stumpel, Servi J. C. Stevens, Alexander P. A. Stegmann, Kristian Tveten, Arve Vøllo, Trine Prescott, Christina Fagerberg, Lone Walentin Laulund, Martin J. Larsen, Melissa Byler, Robert Roger Lebel, Anna C. Hurst, Joy Dean, Samantha A. Schrier Vergano, Jennifer Norman, Saadet Mercimek-Andrews, Juanita Neira, Margot I. Van Allen, Nicola Longo, Elizabeth Sellars, Raymond J. Louie, Sara S. Cathey, Elly Brokamp, Delphine Heron, Molly Snyder, Adeline Vanderver, Celeste Simon, Xavier de la Cruz, Natália Padilla, J. Gage Crump, Wendy Chung, Benjamin Garcia, Hakon H. Hakonarson, and Elizabeth J. Bhoj. Histone h3.3 beyond cancer: germline mutations in histone 3 family 3a and 3b cause a previously unidentified neurodegenerative disorder in 46 patients. Science Advances, Dec 2020. URL: https://doi.org/10.1126/sciadv.abc9207, doi:10.1126/sciadv.abc9207. This article has 92 citations and is from a highest quality peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Ask OpenScientist

OpenScientist Report

Classifications

Inheritance

Mechanistic Hypotheses

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Differential Diagnoses

Experimental Models

Source YAML

References & Deep Research

References

Deep Research

Bryant-Li-Bhoj Neurodevelopmental Syndrome 2: Disease-Characteristics Report

Executive summary

1. Disease information

Definition and identifiers

Evidence provenance

2. Etiology, risk, protection, and gene–environment relationships

Causal factor

Risk factors

Protective factors and gene–environment interaction

3. Phenotypes

Core clinical spectrum

Variability, function, and quality of life

4. Genetic and molecular information

Gene and variant classes

Functional effects

5. Environmental information

6. Mechanism and pathophysiology

Proposed causal chain

Relevant ontology suggestions

7. Anatomical structures affected

8. Temporal development and natural history

9. Inheritance and population

Inheritance

Epidemiology

10. Diagnostics

Recommended approach

Role of other tests

Differential diagnosis

11. Outcome and prognosis

12. Treatment and current applications

Current clinical implementation

13. Prevention

14. Other species and natural disease

15. Model organisms and experimental systems

Patient-derived cells

Zebrafish

Other models and applications

Recent developments and expert interpretation

Principal knowledge gaps

Artifacts